Trying to bulid...

[larsenex]

In my main City I have all the buildings for requirements for the Oxford University. I have a university and such. BUT the key is greyed out. This also occurs for Wall street, which I have a bank as well. Why is it greyed? Has another player built it already?

[GeoModder]

You need a number of universities in other cities before you can built the Oxford. I think five.
You can see the number you need in the textstat popup when mousing over the Oxford icon in the cityscreen.

[xxFlukexx]

Might depend on epic/fast/normal game speed or on map size but in all my normal games on a large map it takes 7 universities to build oxford and 7 banks to build wall street.

Such wonders are called national wonders (as opposed to world wonders) National wonders can be built by every civ so you don't need to worry about 'getting beat' to them.

[Krill]

And then you can only have 2 national wonders per city, except in OCC...

[QuestGAV]

It is indeed 5 banks/universities for Wall Street/Oxford respectively on a standard map.

[larsenex]

Thanks

That explains it. Thanks

[baboon]

Another reason could have been that some other city had build it for one turn before changing to something else. You have to delete it there before you can build it somewhere else.

FYI