I’m assuming there’s a rafter tail above the OSB that’s above the soffit panel. So even if you cut that OSB you’ll have the rafter tail to deal with. How does the roof meet the exterior wall above there? Is there flashing? Could that be where the water is intruding? The sheathing and foam was installed before the siding trim and gutters were there so it’s hard to imagine you getting any easy access unless you’re willing to cut a lot of things off and re build them.