The 19-year-old, one of the stars of the youth team that reached the FA Youth Cup final twelve months ago, came on with 15 minutes to go at St. Andrews in Rovers' 1-1 draw with Birmingham City.
O'Sullivan certainly didn't look out of place, putting in an all-action cameo in the final quarter hour.
"It was a great feeling to come on," said the midfielder afterwards.
"I've been here since I was 15, so to come through the Academy and finally make the breakthrough to the first team, I just can't describe the feeling.
"I was just sitting on the bench dying to go on and thankfully Gaz put me on. He trusted me and said I did well.
"I wish the season wasn't over but I'm just focused on next season now, hopefully I'll break through and can play a few more games."
O'Sullivan has worked under Bowyer at all levels at the football club, starting out with him for the Under-18s at the Academy before both moved onto the Reserves and now the seniors.
Because of this familiarity the midfielder believes that it helped him finally make his senior debut.
"He's had me now from when I first arrived," added O'Sullivan.
"He was the youth team manager, then he was the reserve manager and now he's the first team manager. He's played me at all levels so he must like me a little bit."
For O'Sullivan the season isn't over just yet, he's hoping to be involved in forthcoming matches for the Republic of Ireland Under-21 side.