Hasselbaink, who has led minnows Burton to the top of League One, is expected to be named as QPR boss on Friday according to reports and he will take charge of a team on the up thanks to Onuoha's late header at the Madejski Stadium.
With interim manager Neil Warnock in the dug-out, the west Londoners secured their second successive victory to climb to 11th place, just two points off the play-offs as they look to win promotion to the Premier League.
Reading had chances to win the game but failed to test QPR goalkeeper Rob Green and they were made to pay when Onuoha headed Alejandro Faurlin's corner past Royals goalkeeper Ali Al Habsi's weak attempted save.
Hasselbaink, a former Netherlands international who had a four-year spell with Chelsea and also played for Leeds and Middlesbrough, was given permission by Burton to speak to QPR earlier this week.
The 43-year-old, who was appointed Burton boss in November 2014 and guided the Brewers to promotion from League Two, will replace Chris Ramsey following his sacking last month.