The 28-year-old defender joined the O’s on a six-month loan in July and has quickly become a fan favourite at the Matchroom Stadium.
Coulson departs Cambridge United following 10 years service, having been handed his senior debut in October 2007 in a 2-2 draw against Halifax Town.
The central defender went on to make over 300 appearances for the Club and was rewarded with a testimonial against Tottenham U23s in the summer before making the move to the O’s.
Coulson has made 16 appearances for the O’s, scoring his first goal in the 2-1 win over Haringey Borough in the FA Trophy.
He netted for a second time in the Boxing Day victory over Dagenham & Redbridge at the Matchroom Stadium, and has been part of the O’s defence that has conceded just once in five matches.