Easton, who scored his first goal for the club at Grimsby a week earlier, sent Shane Tudor's right wing cross past Billy Turley to settle this fierce encounter as Oxford slipped to only their second defeat in 11 games.
The victory was no more than Orient deserved as they dominated the first half - but Oxford boss Brian Talbot made two changes at the break and the introduction of Steve Basham and Chris Hargreaves turned their fortunes around and they put the home side under lots of pressure but failed to create too many clear cut chances.
Oxford could not have wished for a worse possible start as defender Jon Ashton suffered a first minute injury following a clash with Jabo Ibehre and was forced to leave the action giving substitute Jude Stirling an early introduction.
As Orient took a vice like grip on the game it was giant defender Leo Roget who rescued the visitors with a goal-line clearance on 16 minutes from Ibehre's shot after Tudor opened up the visiting defence.
Orient's top scorer Gary Alexander and Easton both went close to breaking the deadlock and as the O's turned the screw Turley parried Michael Simpson's header and Ibehre saw his follow up effort scrambled away.
It was all one way traffic and Lockwood went close with a shot from distance before Easton made the all important breakthrough.
After the interval Oxford forced their way back into the game and Lee Mansell almost levelled with a shot from 20 yards that grazed the bar.
As United committed more men forward the lively Basham headed just over from close range but it was seven minutes from time before Oxford forced Glyn Garner to make a genuine save.
The O's keeper comfortably held Barry Quinn's shot as Oxford's late charge came to nothing.