The Lions back-row forward said: "I was delighted to be able to give a long-term commitment to the Ospreys when the opportunity arose.
"The Ospreys are going on to bigger and better things, and I want to be a part of it for the foreseeable future.
"The calibre of the players at the region, the overall quality of the squad and the standard of facilities available to us as players is second to none, and I am convinced we are on the verge of something big."
Wales second-row prospects Ian Evans and Alun Wyn Jones, meanwhile, have also made four-year commitments to the region.
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