Former Carlton forward O'hAilpin spent two years on the Giants' list, but a serious knee injury in 2012 limited him to one game last year and he added a further seven this season to finish his career on 88 matches.
Former Blues teammate Thornton was also thrown a GWS lifeline this year, yet he was only able to make one senior appearance, taking his career tally to 189 matches.
The decision to release the talented Ugle – one of the club's 17-year-old access selections back in 2011 – is more of a surprise, although he only featured in three games over the past two seasons.
South Australian Josh Growden and rookie-listed New South Welshman Joe Redfern have also been delisted after failing to crack the senior team.
"I want to thank all these players for their contribution to the club and wish them all the very best for the future," GWS football manager Graeme Allan said in a statement.
The club also announced on Wednesday that retired Giants Chad Cornes and Dean Brogan will become full-time assistant coaches at the club.
Cornes had been contemplating returning home to South Australia, but has decided to continue his association with the Giants.
They were premiership teammates at Port Adelaide, but a knee injury this season prevented Cornes from adding to his 255 career games, while Brogan made 10 appearances to finish on 193 matches.
"Chad and Dean have helped guide our young players through their first two years in the AFL," Allan said.
"We look forward to their ongoing contribution to the club."