Easter was left out of England's RBS 6 Nations training squad but while Quins were without several players but they were still able to pull seven points clear at the summit with a 30-23 success.
"I thought Nick Easter was absolutely incredible," said O'Shea. "Did England get rid of him too quickly? Yes. Is he finished? No he is not, he is the best number eight in the country.
"He is an outstanding player who started late in professional rugby and he will go on late.
"You need players like him, he's a bit old school but he's bloody talented and hard.
"We're getting back to form but I think we still had a few more gears left in us. We looked a very threatening side today though and deservedly won.
"We knew today was going to be a big day, and to take all the men unavailable out and step up and play like that was good."
However, Quins made a terrible start when Tom Homer scored a try for Irish inside the opening moments with the full-back going on to score all the visitors' points for a first-half 13-10 lead.
Tom Williams, who helped created Tom Casson's first-half try, then scored straight after the break for Quins before having a helping hand in the third try for Ross Chisholm.
Jonathan Joseph went over for the Exiles in the second half but head coach Toby Booth admitted the negatives outweighed the positives.
"We're obviously bitterly disappointed because I thought we were really good for the effort and a couple of key moments have really cost us dearly," he added,.
"People in the changing room are a little bit flat because we were good for it, the endeavour was good, the effort was good but you're playing against the team that is top of the table so it is going to be hard and you have to take your chances.
"They had two chances and took them both, which is credit to them but that is sometimes the cruel master that is professional sport.
"There is no limit to how good Jonathan Joseph and be. I have said before that in my 10 years at Irish he is one of the most talented players I have worked with."
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