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 Russell Slade's side.
Orient offered Allsop a contract extension at Brisbane Road which the 20-year-old turned down.
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.
