The Blues were held to a 1-1 draw against Karl Robinson's side in the first leg of the crunch showdown
Curtis notched the hosts’ opener in the 32nd minute before Marcus Browne equalised for the U's shortly before the interval.
However, Kenny Jackett's side’s had good claims for three penalties throughout the match.
Christian Burgess appeared to be upended by Rob Dickie in the first period, while Curtis looked to be tripped by Dan Agyei with a minute remaining.
All their shouts were turned down by referee Gavin Ward.
And Curtis rued the fact that none were given and travel to the Kassam Stadium for the return clash on Monday all square.
Speaking to Sky Sports after the final whistle, the Irishman said: ‘We know what we set out to do today and wanted to get the win to take it into Monday.
‘It's a shambles. We had three or four penalty shouts. The one at the end, he's just clipped my heels.
‘It's either a yellow card to me (for diving) or a yellow card to him and a pen.
‘It's fine margins and just unlucky. We battered them in the first half and overall we battered them the whole game.
‘We deserved more out of it and it's just the referee and linesmen didn't help me.’
Curtis’ opening goal managed to find its way through the legs of Oxford keeper Simon Eastwood.
But the ex-Derry man was always confident he’d find the back of the net.
Curtis added: ‘I'm a striker and I'm always confident about the goal. The goal never moves and I'm going to take my chances, luckily it went in through the keeper's legs.
‘We've been training really hard for the next three months and we were just unlucky.’