The 24-year-old right-back has put pen to paper on a contract until the end of the season.
Omozusi previously played 62 matches for Orient but his contract was terminated in 2011 after he was jailed for two and a half years for his part in trying to take revenge on a witness in a gang-related murder trial.
But he has been training with his old club since his release from prison in the autumn and O's boss Russell Slade has now added the Londoner to his squad in time for Tuesday's derby trip to Brentford.
Slade told the club website: "Elliot showed a lot of potential and promise during his first spell with the club.
"He's paid his dues now and is more determined than ever to get his career back on track, and we're providing him with the platform to do that."
Orient's chief executive Matthew Porter added: "Elliot made a mistake and has paid his debt to society - he deserves a second chance.
"It's up to him to continue along the right road now and embrace this opportunity."