Open-side flanker Sam Underhill has been ruled out for four months with a shoulder injury.
McCusker made 133 appearances for Scarlets before moving to London Irish last season, where he played 17 games.
The 30-year-old has won 10 caps and joins Ospreys having initially signed for Welsh Premiership club Carmarthen Quins following his release from Irish.
"With Sam's injury and one or two others working their way back to full fitness who are not quite there yet, we felt that we needed someone else in to help us through this period," said Ospreys head coach Steve Tandy.
"We are fortunate to have someone with Rob's versatility and experience available and have brought him into the environment with a view to him helping us prepare for Zebre at home in the opening round of the Pro12 next month."
Ospreys have signed former Scarlets and Wales back-rower Rob McCusker on a short-term contract as injury cover.