Some type of brass ring.

[DARKCYDE]

I've found a few of these.
They are about as big around as an oreo cookie.
Any idea what they may be?

 

[DCMatt]

Curtain rings.

 

[Loco-Digger]

I believe it may have been plated once and is part of some sort of horse tack. See the one in the pic below.

 

[bradyboy]

its a copper washer, used in water meter fittings
Bradyboy

 

[DARKCYDE]

The horse thing actually makes sense. The 2 I have found have been very close to where I also found old horse shoes.

 

[CTwoods]

the enlarged pic shows kinks like it is not solid. Horse harness rings would be solid brass. If it is copper, and has a seam, it may be a gasket filled with asbestos, used on antique cars, or some sort of item that needs a gasket (like was mentioned above)