Thornton has put pen to paper on a two-year deal

The 25-year-old, who passed a medical on Wednesday afternoon, joins the O's on a free transfer.

Boss Martin Ling told the club's website "We've been looking for a creative midfielder and Sean fits the bill perfectly.

"He will fit into our style of play and I believe we have signed a quality player." Ling added.

Irishman Thornton made 77 appearances for Doncaster since signing for the club for a fee of �175,000 from Sunderland.

Thornton was a regular for the Black Cats during their 2002/03 Premiership campaign and helped them win the Championship title two years later.