The siege of Oxford comprised the English Civil War military campaigns waged to besiege the Royalist controlled city of Oxford, involving three short engagements over twenty-five months, which ended with a Parliamentarian victory in June 1646.

The first engagement was in May 1644, during which King Charles I escaped, thus preventing a formal siege. The second, in May 1645, had barely started when Sir Thomas Fairfax was given orders to stop and pursue the King to Naseby instead. The last siege began in May 1646 and was a formal siege of two months; but the war was obviously over and negotiation, rather than fighting, took precedence. Being careful not to inflict too much damage on the city, Fairfax even sent in food to the King's second son, James, and was happy to conclude the siege with an honourable agreement before any further escalation occurred.

Oxford during the civil war The creation of the King's Oxford Parliament in January 1644 placed Oxford at the centre of the Cavalier cause, and the city became the headquarters of the King's forces. This had advantages and disadvantages for both parties; although the majority of citizens supported the Roundheads, supplying the Royalist court and garrison gave them financial opportunities. The location of Oxford gave the King the strategic advantage in controlling the Midland counties, but the disadvantages of the city became increasingly manifest. Despite this, any proposals to retreat to the southwest were silenced, particularly by those enjoying the comforts of university accommodation. The King was at Christ Church and the Queen at Merton. The executive committee of the Privy Council met at Oriel; St John's housed the French ambassador and the two Palatine princes Rupert and Maurice; All Souls, New College, and St Mary's College housed respectively the arsenal, the magazine and an ordnance factory; while the mills in Osney became a powder factory. At New Inn Hall, the requisitioned college plate was melted down into 'Oxford Crowns', and at Carfax, there was a gibbet. University life continued, although somewhat restricted and disturbed; the future kings Charles II (then Prince of Wales) and James II (then Duke of York) were given Master of Arts degrees, as were many others for non-academic reasons. Throughout, both sides employed poor strategies and suffered from weak intelligence, and there was less animosity between the sides than is usual in such wars.

First siege (1644)

Late in May 1644 Edmund Ludlow joined Sir William Waller at Abingdon to blockade Oxford. According to Sir Edward Walker's diary, on 27 May Waller attempted to cross the Isis at Newbridge, but was defeated by Royalist dragoons. The following day, the Earl of Essex and his army forded the river at Sandford-on-Thames, halting on Bullingdon Green in full view of the city. Whilst the main army marched on to Islip to make quarters there, the Earl of Essex and a small party of horse came within cannon shot to make a closer inspection of the place. For a large part of 29 May, various parties of Parliamentarian horse troop went up and down Headington Hill and had a few skirmishes near the ports, although little damage was made on either side—the work at St Clement's Port made three or four great shot at them, driving them back to the main body of troops. The King, being at that time on the top of Magdalen Tower, had a clear view of the troops' manoeuvres.

On 30 May and 31 May the Parliamentarians made unsuccessful attempts to cross the River Cherwell at Gosford Bridge, and Earl of Cleveland Thomas Wentworth made a demonstration with 150 horse troops towards Abingdon, where Waller had 1,000 foot and 400 horse troops. Entering the town, Cleveland captured forty prisoners, but was pursued so heavily they escaped, and although he killed the commander of their party, the Royalists lost Captains de Lyne and Trist, with many more wounded.

Waller finally succeeded in forcing the passage at Newbridge on 2 June, and a large contingent crossed the Isis in boats. The King went to Woodstock to hold council and in the late evening heard news that Waller had brought some 5,000 horse and foot through Newbridge, some of which were within three miles of Woodstock. Islip and the passes over the Cherwell were abandoned, leaving matches burning at the bridges to deceive the Parliamentarians, the Royalists retreated to Oxford, arriving there in the early morning of 3 June. Walker notes that there was not enough supplies to last fourteen days and that if the army stayed in the city and were besieged, all would be lost in a matter of days. It was decided the King should leave Oxford that night: the King ordered a large part of the army, with cannon, to march through the city towards Abingdon to act as a diversion. The King constituted a council to govern affairs in his absence and ordered all others who were to join him to be ready at the sound of a trumpet. After a few hours, the army returned from Abingdon, having successfully drawn off Waller.

On the night of 3 June 1644 at about 9 p.m. the King and Prince Charles, accompanied by various Lords and a party of 2,500 musketeers, joined the body of horse, taking the van which then marched to Wolvercote and on to Yarnton towards Long Hanborough, Northleigh and Burford, which they reached at about 4 p.m. on 4 June. The army's Colours had been left standing and a further diversion was arranged by the 3,500 infantry left with the cannon in North Oxford. The Earl of Essex and his troops had crossed the River Cherwell and had some troops in Woodstock, while Waller and his forces were between Newbridge and Eynsham. Although without heavy baggage, the King had some sixty to seventy carriages, a large troop to have got through undiscovered. The parliamentarian scouting was seriously at fault, unaided by the lack of co-operation between Essex and Waller, it led to a disgraceful inability on the part of two large armies to counter the escape of the King. The escape being discovered, Waller made haste in pursuit, taking some few stragglers in Burford who had "regarded their drink more than their safety". The King and his forces, after assembling in the fields beyond Bourton, continued to march on to Worcester. A letter from Lord Digby to Prince Rupert dated 17 June 1644, gives an indication of the immensity of the lost opportunities; The truth of it is, had Essex and Waller jointly either pursued us or attacked Oxford, we had been lost. In the one course Oxford had been yielded up to them, having not a fortnight's provisions, and no hopes of relief. In the other Worcester had been lost, and the King forced to retreat to your Highness.

Following the unproductive efforts by Essex and Waller to capture Oxford and the King, Sergeant-Major General Browne was appointed command of Parliamentarian forces on 8 June, with orders for the reduction of Oxford, Wallingford, Banbury, and the Fort of Greenland House. Browne was also to select and preside over a council of war of twelve men, and although he greatly troubled Oxford from then on, there were no further attempts on the city during the 1644 campaign season.

Second siege (1645)

In the New Year, one of the first objectives of the New Model Army was the blockade and siege of Oxford, initially intending that Oliver Cromwell and Browne go to Oxford, while Fairfax marched to the west. Fairfax was in Reading on 30 April 1645 and by 4 May had reached Andover, where he received orders to prevent Prince Rupert getting to Oxford. On 6 May Fairfax was ordered to join Cromwell and Browne at Oxford and to send 3,000 foot soldiers and 1,500 horse soldiers to relieve Taunton, which he accomplished on 12 May. The committee had ordered a voluntary contribution from Oxfordshire, Buckinghamshire and Berkshire to raise forces to take Oxford and on 17 May planned for funding Fairfax in the reducing of Oxford, so that "it may prevent all Provisions and Ammunition to be brought in". On 19 May Fairfax arrived in Cowley and made his way over Bullingdon Green and on to Marston, showing himself on Headington Hill. On 22 May he began the siege by raising a breastwork on the east side of the River Cherwell and erecting a bridge at Marston. On 23 May the House of Commons gave the Committee of the Army orders to "make Provision for such Money and Necessaries for the Siege of Oxon, as they have or shall Receive directions for from the Committee of Both Kingdoms, not exceeding the Sum of Six thousand Pounds", having already agreed that £10,000 was to await Fairfax at Windsor, along with the following provision for a siege:

According to Sir William Dugdale's diary, on 23 May Fairfax was at Marston and his troops began crossing the river, the outhouses of Godstow House were fired, causing the occupants to evacuate to Oxford, and the house occupied by the Parliamentarians. On 26 May Fairfax put four regiments of foot soldiers with thirteen carriages by the newly erected bridge at Marston, the King's forces 'drowned' the meadow, fired houses in the suburbs and placed a garrison at Wolvercote. Whilst viewing the ongoing works, Fairfax had a narrow escape from being shot. On the following day two of Fairfax's regiments—the white and the red—with two pieces of ordnance marched to Godstow House and on to Hinksey. The Auxiliaries on duty in Oxford; the Lord Keeper, the Lord Treasurer, and the Mayor of Oxford marched before their Companies to the Guards. In the evening of 29 May a "bullet of ix lb. weight, shot from the Rebels warning piece at Marston, fell against the wall on the north side of the Hall in Christ Church". Meanwhile Gaunt House near Newbridge was under siege by Colonel Thomas Rainsborough with 600 foot soldiers and 200 horse. Next day the sound of firing at Gaunt House could be heard in Oxford and the following day Rainsborough took the house and 50 prisoners.

In the early hours of the morning on 2 June the troops in Oxford made a sally and a party of foot and horse attacked the Parliamentarian Guard at Headington Hill, killing 50 and taking 96 prisoners, many seriously wounded. In the afternoon Parliamentarian forces drove off 50 cattle grazing in fields outside the East Gate. On 3 June the prisoners taken the day before were exchanged and the following day the siege was raised and the bridge over the River Cherwell was demolished. The Parliamentarian forces withdrew the troops from Botley and Hinksey, and also withdrew from their headquarters at Marston and on 5 June they completed evacuating Marston and Wolvercote. The reason for such a sudden withdrawal was that the King, Prince Rupert, Prince Maurice, and the Earl of Lindsey, Montagu Bertie and others had left Oxford on 7 May. In the meantime, Fairfax, who disliked spending time in siege warfare, had prevailed upon the committee to allow him to lift the siege and follow the King. A letter by Fairfax to his father dated 4 June 1645 explains: I am very sorry we should spend our time unprofitably before a town, whilst the King hath time to strengthen himself, and by terror to force obedience of all places where he comes; the Parliament is sensible of this now, therefore hath sent me directions to raise the siege and march to Buckingham, where, I believe, I shall have orders to advance northwards, in such a course as all our divided parties may join. It is the earnest desire of this Army to follow the King, but the endeavours of others prevent it hath so much prevailed.

On 5 June, Fairfax abandoned the siege, having received orders to engage the King and recapture Leicester instead.

Third siege (1646)