Although still to feature in the Guinness PRO12 in an Ospreys shirt, he played a part in each of the three warm-up games and has made a positive impression at the region's Llandarcy Academy of Sport base, earning a prolonged stay.
Speaking after extending his contract, McCusker said:
"I'm really happy to be staying here for the rest of the season and hopefully longer. I've had a great welcome from everybody and I feel I've fitted in to the group well.
"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 that 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. That competition can only help the team."
Head Coach, Steve Tandy, added:
"Rob had already shown that he is a good addition to the squad. He is a leader who has slotted smoothly into our environment. He brings huge experience to the group, not only challenging things but showing real professionalism in how he has quickly got up to speed with what we expect from a player in his position.
"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."
Wrexham born McCusker spent 11 years with the Scarlets, making 133 appearances and scoring six tries, captaining the team between 2012 and 2014. Last October he switched to London Irish, playing 17 games for the Reading based side, 14 in the Aviva Premiership and 3 in the European Challenge Cup.
After representing Wales at age-grade and sevens, he made his full international debut against South Africa in Cardiff in 2010, winning six caps in all.
