The former Scarlets and Wales back-row originally arrived as short-term cover for injured flanker Sam Underhill.
But he has impressed Ospreys head coach Steve Tandy both on and off the pitch amid the region’s unbeaten start to the Guinness Pro12 season.
McCusker, 30, said: “I’m really happy to be staying here for the rest of the season and hopefully longer.
“It’s player focused environment where information flows both ways and, to be honest, it’s refreshing to be in that kind of environment.
“I know there is strong competition in the back-row but I want to fight for a place. I hope that I’ll do enough to get some game time sooner rather than later.”
McCusker made 133 appearances for the Scarlets before moving to London Irish, where he played 17 times last season.
The Wrexham-born forward won six caps after making his Test debut against South Africa in 2010. He joined Ospreys this summer after initially signing for the Welsh Premiership club.
Tandy said: “Rob had already shown that he is a good addition to the squad and brings huge experience to the group.
“Coming into pre-season very late means he has had some catching up to do but we are already seeing big improvements and, as the season progresses, we would look to him to play a big role for us, particularly during international windows.”
