MJO is the Madden–Julian oscillation, the largest element of the intraseasonal variability in the tropical atmosphere.
MJO may also refer to: ISO 639:mjo, the ISO 639 code for the Malankuravan language Mount Etjo Airport, the IATA code MJO Metropolitan Jazz Octet, a group created by Chicago saxophonist and arranger Tom Hilliard
See also M JO, a fashion label created by Taiwanese singer and actor Jiro Wang