Back-to-back draws had left KuPS three points behind their main title rivals with two matches to play, and by only cutting the deficit by a point, their dreams of a third title in a row for the first time in their history will be over if either Inter Turku beat Gnistan or HJK beat VPS later in this round.
AC Oulu, meanwhile, have gone four points clear of Gnistan and put themselves firmly in the driving seat for the final place in next season's UEFA Conference League, while VPS are now officially out of the race for Europe.
The home side went ahead when the match was not even nine minutes old and it was all KuPS' own doing - Samuel Pasanen took a dreadful first touch when receiving a pass out from goalkeeper Johannes Kreidl, allowing Camilo Triana to pick up the pieces and apply a simple finish.
Later in the opening half, Oulu repelled the KuPS response and had a second goal disallowed for offside against scorer Rasmus Karjalainen. The home side were having the better of the chances early in the second half, until KuPS had their best chance when the final quarter was approaching, as the ball fell kindly to Piotr Parzyszek, who forced Miguel Santos into a full-stretch save down to his right.
That proved to be a wake-up call for Oulu, who doubled their lead just a few minutes later, when Karjalainen played a one-two with Lamine Ghezali, who sent the striker through with a well-timed pass, and this time the flag stayed down when Karjalainen found the bottom corner.
Santos and his defence stood firm late on to preserve the clean sheet and secure a memorable victory, a third win in four for AC Oulu, while the Kuopio side have won just two out of nine in the Championship Group.
Follow the dramatic end to the Veikkausliiga title race on Flashscore.