Oldenburg's have very distinctive faces....I don't think that's what she is. Does she has any brands or tattoos anywhere? The problem with "warmblood" is a lot of people market their horses as a "warmblood" when they really aren't. So unless they are registered with their breed organization or have brands, you never really know what they are.
I would definitely not classify her as an Oldenburg. I work with one on a regular basis, and this mare looks nothing like him at all. I would say a TB cross. She could be a Trakehner. The ones I've seen look startlingly similar to TBs.
It is very hard to say, but from the looks of her I definitely wouldn't call her a "true" Oldenburg, meaning a full Oldenburg. I could possibly see her being an Oldenburg/TB cross, possibly a Trakehner, but if I had to guess I would just say TB - I have seen many TB's built alot like her. The problem with warmbloods now a days is just about everything is considered a warmblood - such as TB crosses and even draft crosses. Yes, it is true they are considered WB's, but when I say WB I'm talking about full Warmblood breeds and not crosses, so it is just so hard to tell what is a true WB and what is a WB cross anymore without papers, tattoos, or brands.