Hoolahan struck in the 87th minute as the visitors gained a deserved victory after dominating the second half at Boundary Park.
Blackpool came into the semi-final first leg after seven straight wins which put them close to automatic promotion, but the Latics were the better side before the break.
Top scorer Chris Porter headed at the keeper from close-range and left-winger Chris Taylor was causing problems with his trickery on the ball.
Blackpool striker Keigan Parker tested Les Pogliacomi in the Oldham goal, but John Sheridan's Latics made two chances just before the break as Paul Warne blazed off-target and Sean Gregan drove just wide from the edge of the box.
Blackpool, however, turned the tables and Parker again tested Pogliacomi before they went in front after 52 minutes.
A Robbie Williams corner was thundered in by defender Shaun Barker as he arrived to meet the ball six yards out.
Blackpool created plenty more openings for Parker and his strike partner Andy Morrell but Oldham's substitute keeper Alan Blayney, on for the injured Pogliacomi, kept them at bay.
The equaliser came on 75 minutes as Porter was felled in the box by Michael Jackson and Andy Liddell fired the penalty inside the right-hand post despite Paul Rachubka diving the right way.
Oldham would have been happy with a draw, but Hoolahan denied them when he raced on to Andy Morrell's pass and rounded Blayney before slotting home from eight yards.
Blackpool boss Simon Grayson said: "I am happy with a 2-1 lead halfway through the tie and I thought some of our football was very good, but it could have been all over by now."