McLean beat off strong competition from Niall Harty.
O’Mahony is the fourth independently elected editor.
Flood ran on a platform of expanding Trinity Ents.
Aimee Connolly will be the next TCDSU Education Officer.
James Cunningham was elected in one of two uncontested races this year.
The result comes as a surprise, after polls last week suggested option four would prove most popular.
There was a 20.8 per cent increase in voter turnout compared to last year’s elections.
In a statement, the candidate rejected allegations of sexual harassment made against him.
Queen's University Belfast Students' Union (QUBSU) have pledged their support to striking academics.
The policy will see 170 toilets designated as gender neutral and the introduction of transgender changing facilities.