O'Kearney, 20, impressed the Saints with his professionalism and finished second in Sandringham's best and fairest while playing his 18 games as a half-forward/midfielder.
However, his relatively slight frame (181cm, 72kg) made it tough for him to break into the senior side.
"Nick's attitude has been exemplary in his time at the Saints," St Kilda coach Alan Richardson said.
"Unfortunately he was unable to force his way into the midfield group at AFL level. Nick is a great young man and we wish him all the best for the future."