O'Mara spent six years at the Gills having joined them at the age of 13. He made his professional debut during the 2017-18 campaign when he came on as a substitute away to Blackburn Rovers.
The defender started in a 1-0 EFL Trophy win at Colchester United before making his full league debut at home to Portsmouth.
His final Gillingham appearance came in a 7-5 Checkatrade Trophy victory over Reading under-21s in November 2017.
O'Mara signed for Invicta in October 2018, having had two loan spells at Cheriton Road, and has since made over 70 appearances for the club.