O'Brien has been equally prominent for the Dubliners in the Magners League this season with three tries in 12 appearances.
And he will have the opportunity to claim yet another piece of silverware when Leinster travel to Munster on Saturday for the Magners League Grand Final.
O'Brien was one of three Leinster players nominated for the award and along with teammates Jamie Heaslip and Isa Nacewa he saw off the competition of Northampton Saints prop Soane Tonga'uiha and Stade Francais Sergio Parisse.
"It's been a fantastic year for me, and more importantly for Leinster," said O'Brien.
"It's great to be presented with an award like this, but it has more to do with the team than it has to do with me.
"Hopefully, this is only the start rather than the end of our success in Europe, and of course, we've got another trophy to aim at this weekend."
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