I'll start. I'm sure there are several ways to tackle this, but here is what I would do.

Remove another OSB panel on each side. You need to get to solid wood on each side of the damaged area.

Cut away the rotted material. When the wood is solid enough to take a nail or screw you've cut away enough.

Cut a replacement piece to fill the removed rotted board.

I would add a new board above and below the original. Have the new wood extend at least half the distance to the gap in the old board. Example: If you cut away 4 feet I would use at least an 8 ft new board above and below.

I prefer screws but nails are fine. Use a lot :)