Stevens has been out since January with a calf complaint, but stepped up his recovery with 45 minutes for the U23s in the international break before being replaced as planned at the break.
Colchester actually took the lead in the Bramall Lane sunshine, Chay Cooper curling a free-kick that caught out Jake Eastwood.
Just over 30 seconds later the Blades were level, however, when Osula took advantage of his man going to ground injured to score at the second attempt after his initial effort was saved.
And just 19 seconds after play was restarted again, the Blades had the chance to go ahead when Stevens won the ball back, Osula was sent clear and then brought down as he looked to convert.
Osula grabbed the ball and duly sent the goalkeeper the wrong way from the spot, in front of the watching Blades boss Paul Heckingbottom.
Hassan Ayari got in on the act to make it 3-1 in the first half, with a composed finish after a nice backheel from Sydie Peck, before Osula sealed his hat-trick with another penalty in front of the Kop.
The young striker, who recently made his debut for Denmark’s U19s side before making his professional bow as a young substitute in United’s goalless draw at Blackpool, then made it four after rolling his man expertly and then finding the bottom corner.