The former West Brom youth player joined the O's in July after being released by Millwall and went on to make 24 appearances for the club.
The npower League One side offered Allsop a contract extension at Brisbane Road which the 20-year-old turned down.
Orient manager Russell Slade told the club's official website: "We're disappointed to be losing Ryan because we made him three offers and met every demand he made.
"He arrived at the club having never made a senior appearance and we felt we did wonders for him here."
Teenage striker Ryan Brunt has also left the club after his loan spell from Stoke came to an end.
- Sports & Recreation