Brown started the last two autumn internationals on the wing for England but was back in his familiar position for Harlequins and in soaking wet conditions was superb under the high ball.
He helped Harlequins win 19-8 and inflict on Northampton a fourth defeat in their last five home games as the Saints tumbled out of the Aviva Premiership's top four.
O'Shea said: "He liked being back at full-back. He wants to play full-back for England, but he will play anywhere for England, he would play at prop for England. He was outstanding for us today."
The Harlequins boss revealed Lancaster spoke with Brown and O'Shea ahead of their home Heineken Cup game against Zebre.
"He came over to watch the team run and we sat down and had a chat," said O'Shea.
"Browny wants to play full-back for England, and I always tell him my selection would be different, but as long as he is playing for England he doesn't give a damn. It is a fabulous position for England to be in with Browny, Alex Goode and Ben Foden, they are pretty rich in that position. Lancaster wants to see players putting their hand up for selection.
"It was a great team performance today, Nick (Easter) will keep knocking on the door, and he is in incredible shape. I just saw in our senior guys through the week a mentality that they wanted to come here and make a statement. That is 14 wins out of 17 in all competitions and we are playing poorly I am being told."
Northampton director of rugby Jim Mallinder criticised the decision not to go to the television match official to review a disallowed Ben Foden try. Foden's first half score was ruled out because of a forward pass from George Pisi, but replays showed it was a very tight call.
"It is ridiculous, why have technology and not use it?" said Mallinder. "The players did ask and he (referee Wayne Barnes) said because he had already called the forward pass then you don't go upstairs. You would love to get five or maybe seven points but you have to accept those things happen, and we need to look at our parts of the game we didn't get quite right today."
Northampton turned down two first-half penalties and instead kicked to the corner, with both attacks coming to nothing, but Mallinder said: "I back our players there. They weren't guaranteed penalties and you trust the players to make the right decision - I will certainly support them on that. Likewise with a few minutes to go to take the penalty and to try to come back and get the try was the right option, so it was disappointing to concede that last penalty."
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