The 26-year-old striker moved to Brisbane Road in January to build up his fitness and made 13 appearances for the O's, scoring three goals.
But Lions boss Kenny Jackett has brought him back to south London to bolster his attacking options ahead of a hectic end to the season for the FA Cup semi-finalists.
Orient boss Russell Slade told his club's website: "I want to thank Shaun for all his efforts in an Orient shirt.
"He's been a real success during his time here and aside from some terrific performances on the pitch he has been an excellent figure in and around the changing room."
- Sports & Recreation