Between 1943 and 1957 O'Sullivan designed commemorative stamps for the Irish Post Office. He created commemorative stamps which depicted his paintings of Douglas Hyde and Sir William Rowan Hamilton in 1943, Edmund Ignatius Rice in 1944, and in 1966 O'Sullivan's paintings of the seven signatories of the Irish proclamation, Éamonn Ceannt, James Connolly, Joseph Plunkett, Patrick Pearse, Seán MacDiarmada, Thomas MacDonagh and Tom Clarke, were featured on stamps produced by An Post.
