The 31-year-old originally arrived at Liberty Stadium on a short-term deal in summer, 2016.
But after leading them three times in 11 appearances last season, the six-times capped player has now signed a two-year contract extension.
McCusker made 133 appearances for Scarlets before moving to London Irish, where he played 17 games in 2015-16.
He initially joined Ospreys after signing for the Welsh Premiership's Carmarthen Quins.
Ospreys will keep former Scarlets and Wales back-rower Rob McCusker until the end of the 2018-19 season.