Advice on wall repair

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 :)

Thank you Steve!

I was anticipating (dreading) needing to remove more of the OSB.

Is this what you are describing?

Blue is the existing upper board with the hatched section representing the rotted part to be cut out. The red would be two new, longer boards to span the gap where the rotted wood was removed - both above and below. Do I leave that section where the rotted wood was "open"?

Barb- since you have to do this repair anyway, you might want to consider a larger window. It’s pretty simple to frame in, although you would need to do the ‘outside work’ too. That said, what’s above that ceiling? Just the roof? Or another story? What’s the roofing material? You should have no problem replacing that crossmember, like for like. I don’t think it’s a header, but more of a nailer (which is good!). I don’t see any vertical supports, so I’m guessing those are just there to attach wallboard, not really for structural support. 

Given that it’s been standing on its own, there shouldn’t be an issue. As stated, you need take the OSB boards out on either side and expose the rest of that damaged board. You need to keep going until you get to no more rot/discoloration and then you want at least 24 inches of overlap for your sister, unless you use metal straps to support it. There are actual code sections that will tell you how many nails, what pattern, etc, but the reality is you just need to make sure that you have really solid overlap and nail it all together. If you go far enough sideways, you may find where the vertical members are . Then you could just replace the whole thing. Wall board is easy to put back up. FWIW, nails are better than screws since they’re stronger in shear, but either will work fine here. 16d nails if you go that route, IMO. 

Barb - Yes! that is what I was trying to describe

good suggestions barb. i dont envy you. if you remember i had some rot issues i had to deal with myself lately.

Hmm.  It seems like there should be more studs (on 16" centers) in that wall.  Is there some sort of large beam at the top of the wall hidden from view?  

At a minimum, you are going to want to frame in the window better than you currently have for the AC unit.   And while you are at it you might as well make the window larger and at the right height too.   By doing something like the examples below, you can use the window framing to support what looks like a nailing board to me (it is weird that they are horizontal instead of vertical).  I would probably just cut out the rotted section and connect it to the side of the vertical window framing.   You will need to do the same for the other horizontal boards too, though you may need to splice in an extension once all of the rot is gone.  

 

Pole barns are tricky to retrofit with stuff on the walls since they don't typically have any structure in the wall except for the occasional nailer. 
You can do the grafting or if you want a window, build up a bit of support underneath like Nathan shows. Since there is no roof load to support you can skip headers and just secure to that top board with some rafter type plates and then frame the opening with some 2x's. It is nice to double them up around any opening to get the extra beef for installing any trim.

Thank you for all the information! For that rotted board, we'll remove the other panels and see how far we need to go to add new support. Appreciate all the suggestions. 

For perspective, this is a pole barn - wood "structure" with insulation and then the metal shell.

This issue is on the back side. The idea of a window to replace the AC was intended to make it "easy" (insert a sad chuckle...nothing has been simple lately) to deal with the opening. I'm wondering if I can get a panel to replace that section as I would be fine with no window on that wall (already have the three windows in that room that show on the picture). This would allow me to repair the inside and then make sure of no future leaks. Not sure how to do a "patch" otherwise that would be nice and waterproof?

Best would be to source another panel and hope it can sun-fade to eventually look seamless.
If you have the cutoff scraps from when it was built, you could shingle them but I'd look kinda hokey.
Any other covering for the hole will have to extend  from under the eave to below the lower edge of the hole to keep water out.

That side faces the woods on our property so I'm not overly concerned with perfect color match. This was built in the 90s so no leftover pieces. Will be interesting what we can find to fit as I have no idea on if/how the panels are universal or each brand differs (hope not the latter).

Think we are headed to pick up what supplies we can and then will tackle the additional "tear down." Anyone bored in SW Missouri is welcome to join the fun. 😬😉🤣

I missed/forgot that this is pole barn construction.  If one side of the window is not against a post, the window will need to be supported like this and you can tie in the repair into window header

Lazyman - hoping I can source a couple metal exterior panels so I can just close this in and avoid dealing with a window. Actually think I'll like the wall surface better. 

Working to hang things in a pole barn has-been challenging. 

Just more food for thought,  
you could put in a window and paint the back side black, then cover the full wall on the inside. 
A window facade....

Only a few metal paneling Mfg's in the states I'm aware of (spent time trying to source an older style for damaged repacement). They seem to be fairly standard, at least for the common styles.
Can you post a picture of the panel style?

You could just cover the hole with plywood on the outside and put some fake shutters over it and close up the wall on the inside.  On my garden shed I found an old salvaged wooden window, painted it and just hung it on an outside wall to dress up the shed. I also put a flower box below it. You can paint the inside of the glass black or add some reflective film to hide the fact that it is a fake window.  

I like to take advantage of problems when I can. Instead of paint or film, A simple poster will make any potential burglars think twice!

Here's the panel

And here's the AC unit - of course, impacts two panels.

barb should be easy enough to get a new panel and just cover up that opening since you dont want a window anyway ?

That's the same profile I have now.

Lots of suppliers, this popped up in MO so probably just down the street right?

I got single pieces from a local home depot, 8' lengths off the shelf.
Anyway, all these suppliers use a small set of dies to form flat rolls of painted metal into the corrugated stuff so maybe only a 1/2 dozen styles.

You can remove the screws and just lay new panels over the old if you don't want to mess with removal.

EDIT:
Oops! Just realized you have three minor ribs between the peaks. Still could work if the edge profiles are the same, but you'd have to remove the two originals.
Anyway, you should be able to take you photo and ask some of the building supply places if they can find it.

Barb, that type of metal panel is called R panel. There are a few manufacturers and although the exact profile will be close, they may not be an exact match. But, it will be very close. They are 3 feet wide and if you want to close the opening completely, buy a full panel and it will blend in, except for the color.
All of our buildings have the same R panel exteriors and roofs and I have helped in building some of them.

I am adding an extension to one this (or next) week. The roof extension will slide in under the existing one. Because of different manufacturers, it might not be a perfect match.
Part of the roof is going to cover the slab.

Close-up of the rib pattern although mine only has two minor ribs between.

At the bottom left, you see a screw that locks two panels together lapping over at the ribs. That requires a screw called (of course) a lap screw. The remainder screws are a type that screws metal to wood.
You may be able to use the same screws or buy them in small quantity at a small hardware store like ACE.

Take your pictures to a metal building contractor and he can advise you or help you out sourcing material.