An outside faucet leaked for some time inside the exterior wall before it was noticed. Now on investigating I find the OSB sheathing to be rotting in this area. The outside wall is synthetic stucco and interior wall cedar. My access to the area in question is a 14" x 14" access panel and 3" along the floor where the base board was. The rotting area is between two studs and extends from the access panel to the floor(about 40"). I would like not to replace the entire sheathing nor make any more holes in the cedar. As it is there is no visible cosmetic damage from inside the room or on the exterior stucco. Obviously I fixed the leak and will dry the area.

Is there anything that can be done to repair the rotted OSB (the studs are fine)? Do I need to do anything?

If You Can't See And Damage Id Just Leave It Alone, I Would However Spray Some Kilz Primer In The Opening To Kill Any Mold That My Grow From It Being Most. Do This After It Has Dried.
It Really Sounds Like It Will Be Tough To Get To The Osb, So Unless It's a Problem To Just Leave It, Just Keep The Mold From Growing.
Im Sure There's Other Products You Can Use To Kill It If Its Even In The Wall, But If You Paint The Inside With It, It Should Be Able To Grow On The Kilz Primer