Derby County are set to make a number of changes to their squad this summer.
O'Brien was deemed surplus to requirements at the iPro Stadium and he admits that it has been hard to take in the news.
"I am gutted to be leaving, but if Steve McClaren does not see me in his plans then that's football," he said.
"Derby County is close to my heart. The staff and players at Derby over the years have been excellent with me. I cannot thank them enough."
The 22-year-old still holds a close affinity to Derby after the club helped him get through a number of injury problems earlier in his career.
O'Brien has found it difficult to stay off the treatment table throughout his career so far and that may be part of the reason why he has been let go.
The Irish defender has a lot of natural ability, but his inability to stay fit has caused the club to send him out on loan this term.
O'Brien has been playing for Motherwell all season, but he has obviously not done enough to convince McClaren that he has a long-term future at Derby.
The Rams are determined to get promoted next season and O'Brien's dismissal is the first sign that McClaren is set to be ruthless this summer.