The 27-year-old Frenchman was a free agent after leaving the Seagulls last month and has agreed a contract until the end of the season.
"Romain is a great signing for us, he has an excellent attitude and will give us versatility," O's manager Russell Slade told the club website.
"He's played in midfield or across the back four at Championship level so he's got good experience and he can score goals too.
"I've admired him for a while since his time at Dagenham & Redbridge and if he impresses during the time he is here then we will do all we can to keep him at the football club.
"We are starting to get some depth in the squad and with players coming back from injury we are looking strong for the final third of the season."
- Sports & Recreation
- Russell Slade