The closest Northern Ireland would come to a third in the opening 25 minutes of the second half came when Iker Alvarez, who had previously made a smart save from Jamal Lewis, almost allowed Spencer's cross to sneak in at his near post.
Having opted against making changes at half-time, O'Neill would make five between the 60th and 68th minute, including an international debut for 21-year-old Nottingham Forest defender Aaron Donnelly.
As the small crowd began to call for Bradley to shoot each time he touched the ball, it was replacement Dale Taylor who tried his luck from distance but Alvarez was always equal to the effort.
With O'Neill's replacements beginning to lift the tempo, it was another second half introduction, Ross McCausland, who next had a go but his acrobatic effort never had the accuracy to trouble the keeper.
There was to be no hat-trick with Bradley, who had smashed one last shot against Alvarez, replaced by Caolan Boyd-Munce with a quarter of an hour to go.
As the game entered its final ten minutes, Eric Vales chased back well to cut out McCausland's square ball that would have left only a tap in for Magennis. The Rangers man will likely reflect, though, that he would have been better going for goal himself.
In the end the third just would not arrive. Not for the first time, nor likely the last, it was Conor Bradley who was to prove Northern Ireland's match-winner.