DOS 20 may refer to IBM OS/2 versions:
OS/2 2.0 ("DOS" version 20.00) OS/2 2.1 ("DOS" version 20.10) OS/2 Warp 3 ("DOS" version 20.30) OS/2 Warp 4 ("DOS" version 20.40) OS/2 Warp 4.5 ("DOS" version 20.45)
See also DOS (disambiguation) DOS 2 (disambiguation) DOS 10 (disambiguation) DOS 30