Here is a look back at a selection of players who have turned out for both sides down the years. How many of them do you remember?
9. MARTIN LING
Martin Ling made just three appearancea for Mansfield on loan from Southend in 1991. But the midfielder later played 148 times at Orient and went on to manage them too. Photo: Pete Norton
10. ADAM BARRETT
A popular no-nonsense centre half, he began his career at Orient but, unable to break into the first team, moved on and was with Stags from 2000 to 2003 before long spells with Bristol Rovers, Southend United (twice) and Gillingham. He is now a coach with Millwall. Photo: JPI
11. CALVIN ANDREW
Calvin Andrew had a loan spell at Orient from Crystal Palace in 2012. He only played 15 times for Stags in the league in 2013/14 and only scored once - but that will be remembered forever as it was the winning goal away at local rivals Chesterfield. Photo: Pete Norton
12. IYSEDEN CHRISTIE
Sometimes brilliant, often frustrating, striker Iyseden Christie joined Stags on loan from Coventry City in 1997. Signing a permanent deal, he played over 90 league and cup games for Stags before joining Orient for £40,000. But he then moved back to Mansfield, scoring 26 goals in 64 games and helping them to the 2004 Third Division play-off final. But, after defeat by Huddersfield, he was released Photo: JPI