Wellens has been appointed as the new head coach of Orient.
He was the manager of Salford City and Doncaster.
Orient is currently 20th in League Two with 35 points.
"I've always enjoyed bringing my teams here, and the potential to improve here is clear to see," he told the club's website.
I want the fans to buy into what we are about and improve the match experience for them.
Football is about having fun and we want to give them something to shout about.
After the club was demoted to League Two, Wellens was dismissed.
The season was called off in March due to the coronaviruses, but he won the League Two title in the year after.
He won the delayed EFL Trophy final in March of 2021.
Since beating Swindon on December 7, the Orient are winless in 14 games.
Martin Ling, director of football, said that he was delighted to have him on board.
He was one of the potential candidates we spoke to last summer, however the Doncaster job came up and he let us know he wouldn't be put through to the next round.
The way he plays the game is an attack minded coach with a team on the front foot, his teams work hard, both offensive and defensively.
The roles of Matt Harrold and Brian Saah will be the same.