O'Sullivan strikes to secure replay
Derry City 1 Cork City 1
Derry City and Cork City will have to do it all again on Monday night, at Turners' Cross.
Both sides gave their all at a rain-swept Brandywell, but they could not be separated.
Derry took the lead on five minutes as Ryan McBride headed home his first goal of the season, powerfully making no mistake from Patrick McEleney's corner.
Cork responded well and they went close to levelling on 28 minutes when Billy Dennehy's shot took a deflection off Aaron Barry, before flashing wide.
They equalised on 49 minutes after a mistake by Dean Jarvis let in Karl Sheppard down the right. His shot was kept out by Gerard Doherty, but Mark O'Sullivan fired home the loose ball.
Cork went close again on 58 minutes, with Sheppard's near-post header just wide.
Derry, though, came back into it, with McEleney long-range strike forcing a save from Mark McNulty, who denied the same player again late on.
Derry City - Doherty, McLaughlin, McBride, Barry, Jarvis; Dooley (Curran 80), Cornwall, McNamee, McEleney; Quigley (Morrison 65), Timlin.
Cork City - McNulty, Bennett, D Dennehy, Murray, (Dunleavy 76) Gaynor; McSweeney, Miller, Buckley, B Dennehy; O'Sullivan, Sheppard.
Ref - P McLaughlin (Donegal).