O’Brien underwent knee surgery this week and has already been ruled out of the RDS showdown with Jones’ men.
Cian Healy is also a major doubt and faces a late fitness test, which will make or break his hopes.
Jones reckons the duo’s absence would be a major blow to Leinster’s hopes of securing a historic Heineken Cup and PRO12 double but still expects a titanic struggle in Dublin.
He said: “It’s a massive loss, they are two of Leinster’s big ball carriers. I’m sure Cian could do a job at 80%, it’s a one off game and people rise to the occasion. But with the strength in depth of their squad they will still be strong irrelevant of who plays.”
The Ospreys demolished Munster at the semi-final stage and have already racked up two regular season victories over the kings of Europe.
But the Wales second-row insists previous results count for nothing and has urged the Ospreys to replicate the physically dominant performance that overwhelmed Munster two weeks ago.
Jones said: “All those wins and losses in the past are gone, they are irrelevant now. Leinster are a different animal, especially after what they achieved this year. It’s a separate game, a big final and one that’s away from home. The Munster game was really pleasing. We haven’t been as efficient or as clinical as that in a long time. We want to keep pushing that, starting on Sunday, and doing it on a consistent basis is the next step.”
The Ospreys have made just one change to the side that romped past Munster with Rhys Webb replacing Kahn Fotuali’I, who passed a late fitness test to line-up on the bench.
Head coach Steve Tandy added: “Kahn’s been terrific for us and it was disappointing that we had to leave it late but hopefully he can have an impact. We’re quite blessed; Webby coming in doesn’t make too much of a difference.”