Stuart Lancaster preferred Phil Dowson and Ben Morgan as his options at the base of the scrum in this year's Six Nations - as well as the summer Tour to South Africa.
But O'Shea believes Easter, 34, still has plenty to offer England despite his age, citing the example of New Zealand lock Brad Thorne, who helped the All Blacks lift the World Cup last season, aged 36.
"To me he is the best No.8 in England. I don't care whether you are 19 or 34, if you are the best you are the best," said O'Shea, who guided Harlequins to the Premiership title last season.
"For the last year he has been playing absolutely unbelievable rugby for us and long may that continue.
"And it will because he has got the bit between his teeth.
"I thought he was magnificent, taking the fight to them. He drives the group; he is incredibly ambitious and proud.
"He has not written off his international career. (New Zealand's) Brad Thorne was older than he was playing in and winning the last World Cup. If he can keep himself in shape and plays the rugby he does, he will knock his way back in there."