The defender was a key member of the Hoops' Under-19 side which clinched a league and cup double last season.
Despite their success, however, O'Donnell was among a number of players whose services were no longer required.
But he has penned a one-year deal with the Jags in a bid to kick-start his career and hopes the time spent working with former Celt McNamara will help him on the road to stardom.
"I watched the manager when I was younger, he was a hero and somebody I looked up to and respected for what he achieved in the game," O'Donnell said. "If I was ever lucky enough to have half the career he had then I would be happy.
"It is difficult to get a game just now because we have good players in every position, but I am enjoying it. The manager and Simon Donnelly have been great with me. It is a great club and hopefully I can do well here."
O'Donnell looked set to follow in McNamara's footsteps at Parkhead after impressing at youth level, but his Hoops ambitions came to a premature end in the summer.
O'Donnell bowed out at Celtic in style with an Old Firm cup triumph on a night that paved the way for a switch to Firhill. He said: "I played in the Youth Cup final against Rangers and the gaffer and Sid were there. I think they were quite impressed, but they didn't pursue anything at the time because I was possibly going to England. That didn't work out, so they got in touch and I trained for a couple of weeks.
"I played in a couple of games and, thankfully, did enough to get a contract. Now I just want to make my debut."
O'Donnell's Hampden success was to be his farewell to Celtic as he left to embark on the next stage of his career.
Predominently a right-back, the youngster will provide cover across the back four and the 18-year-old admits the lure of first-team football was too great an opportunity to spurn.
O'Donnell said: "I was disappointed to leave Celtic but, to get an opportunity to play regularly was the most important thing.
"At Celtic, I would have been in the reserves and not playing competitively. I need to be playing first-team football and, hopefully, that is what I can do at Thistle."