Post office at ORD?
 

I need to mail a letter from within the US. I will be flying in to ORD on LH, and transiting to a UAX regional flight. Is there anywhere along that route at ORD where I can buy a stamp and mail a letter?

Yes, there's a post office in T2, up front -- I think near the children's play area. There's a chance your UX will leave out of one of the nearer T2 gates. If it's T1 and you have a stamp (remember, it's now $0.41), there's a postbox near the duty-free in T1, concourse B.

Just be aware of its hour of operations at the T2 airside post office facility, which are:

Thanks. Since I don't have a US stamp, I will pretty much have to go to T2, right? Or do they sell stamps at any of the airside shops in T1?

The gift shops might have them (to go along with post cards), but I cannot confirm.

41 cents for F domestic letter starts this monday:-:

edited for typo
--

Isn't that 41 cents?

Yes.
http://www.usps.com/prices/

There are also stamp machines located on the outside of the post office which you can utilize if they aren't open (IIRC, the machines accept US currency only, no credit cards). Mail pick-up is at 5pm.

Also, it's on the side of T2 which is nearer to T3 - look for the kiddies' play area & it's in the same general area.