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."

They say the arrests happened when protesters refused to leave land owned by the pipeline company.
A spokesman said the latest arrests brought the number detained since August to almost 700.
They came after the US Army was ordered to allow the construction of the final section of the Dakota Access Pipeline.
Native Americans and their supporters have protested against the project for months, and have vowed to fight on.
The Standing Rock Sioux Tribe say the final section - under Lake Oahe, a reservoir on the Missouri River - would contaminate drinking water on their land and damage sacred burial sites.
Morton County Sheriff's Office spokesman Rob Keller said the latest arrests were made after demonstrators moved from their existing camp on flood-prone ground to land owned by the pipeline company, Energy Transfer Partners.
He said no injuries had been reported during the operation. Protest leaders could not be reached for comment.
The $3.7bn (Â£2.8bn) pipeline is designed to transport about 470,000 barrels of crude oil a day across four states, from North Dakota to a terminal in Illinois, where it can be shipped to refineries.
The US Army Corps of Engineers, which has approval authority, decided last year to explore other routes for the pipeline amid huge protests by the Standing Rock Sioux Tribe.
But earlier this week, acting Secretary of the Army Robert Speer ordered the corps to allow the work to go ahead.
President Donald Trump recently signed an executive order signalling his support for the pipeline.
Ospreys have signed former Scarlets and Wales back-rower Rob McCusker on a short-term contract as injury cover.
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Police in the US state of North Dakota say they have arrested 76 people protesting against a controversial oil pipeline.