Michael John O'Hara (22 February 1933 — 24 November 2014) was a British geologist who specialised in igneous petrology.

Born in Sydney, Australia, and raised in the UK, O'Hara began his geology studies at the University of Cambridge, where he spent the period 1952-1958, as a member of Peterhouse college, and was awarded both his undergraduate and PhD degrees.

O'Hara's made fundamental contributions to the understanding of the origin and evolution of basalts, and their chemical signatures.

[10] He also went with Eric Langmuir "to investigate a report that Eric's father had given of some rocks on Beinn Trilleachan that he had spotted when fishing on the loch and which might be of interest", where they made the first ascent of Sickle and of Spartan Slab (Spartan being another 3 star classic), and in doing this they established the first modern rock climbs on the Etive Slabs.

[11] Another 3 star classic route that he pioneered, after initial exploration with Langmuir, was Minus One Direct on the north face of Ben Nevis.