Since joining the Club at the start of August, Bartley has overcome the initial injury problem that hampered his pre-season preparation and has made one start and three sub appearances with the O’s.
The 27-year-old will now remain with Russell Slade’s side until the new year and the manager believes that the O’s fans are yet to see the best of him.
Slade said: “We are delighted to keep Marvin on board, particularly as at one stage it looked like we might be losing him. He had a difficult start with us as he wasn’t fully fit due to the injury he picked up at Burnley in pre-season. However, in training and during the limited minutes that he’s had on the pitch, he’s shown himself to be a very capable player.
“Trust me, there’s a lot more to come.”