Portadown crashed to a hefty defeat against Drogheda at United Park which has severely damaged their chances of making the Setanta Cup semi-finals.
James Marks challanges Paul Keegan at United Park
Two quick goals from Paul Keegan and Shane Barrett set the scene for the game which was one-way traffic.
O'Brien scored either side of the break to move his side into an easy 4-0 lead before missing a 69th minute penalty.
He saw his spot kick rebound clear of a post after Kyle Neill was shown a red card for a foul on Shane Robinson.
Drogheda were quickly into their stride and two goals in two minutes had Portadown reeling.
It was a Barrett deflected shot that fell into the path of Keegan who fired home from 15 yards.
Before Portadown had time to settle they found themselves two down when Barrett curled a beautiful right-footed shot into the left-hand corner.
The game was over a contest in the 33rd minute when Declan O'Brien slotted home after Portadown failed to clear a free-kick that bobbled around their penalty area.
It took Drogheda 10 minutes to stretch their lead in the second half with an O'Brien thunderbolt from 25 yards thudding the underside of the crossbar and adjudged to have crossed the line.
It got worse for the Ports in the 69th minute when Neill was given a red card after he upended Robinson as he bore down in goal.
O'Brien's miss spared more embarrassment for Ronnie McFall's side.