El Dee already covered it, but since I wrote this up I'll post it [emoji3]
Is there any framing (2x boards) at the outside edge of the cutout in the osb? Not a must-have, but it makes things easier.
This is what I've done, at both our home and camp:
Get a little piece of plywood that's maybe 3" wider and taller than the full U cutout area.
Drill a hole in the plywood that's maybe 1/2" larger diameter than the width of the pipe where it goes through the osb (Some dollar stores, like Real Deals, may have hole saw bits, otherwise maybe go for harbor freight, get a set of these using the 20% off coupon they always have available:
3/4 In - 5 In Carbon Steel Hole Saw Set 18 Pc) A jigsaw can also be used, after drilling a pilot hole to get the blade through.
Cut the plywood, going through the middle of the hole you drilled, using a circular saw. If there's enough room to jigger the upper piece in, go for a horizontal cut, otherwise you'll have to go with vertical.
Shove the pieces into place, attach each piece with 4 - 5 screws into the osb.
Fill the gap between the hole in the plywood and the pvc pipe with some steel wool (to keep mice out).
Optional: caulk the joint between the two pieces of plywood, and/or along the outside edge of the patch.
Once the hole is patched, your choice if you want to add a little insulation. In your climate, likely not necessary once the hole had been patched.
Alternate, if there's only a couple inches from the top of the piece of pvc to the top of the cutout:
Put in a couple inch piece of plywood horizontally, filling the top part of the hole.
Put in a vertical piece, overlapping the first one, fill the right side of the hole.
If the left side space is small, stuff with steel wool, else do another vertical piece (on the left), then stuff everything with steel wool,
(The steel wool will also provide a support for the spray foam El Dee mentioned)
