Is it "transferrer" or "transferer"?

The word transfer is stressed on the first syllable as a noun, and either the first or second syllable as a verb.

In general, you double an "r" at the end of a word when the second syllable is stressed (referrer, referred), but not when the second syllable is unstressed (caterer, catered).

Since transferred is treated as though the second syllable is stressed, presumably transferrer should be as well.

Google Ngrams shows an overwhelming performance for transferor, both in the U.S. and in the U.K.

You could argue that transferor has a specialized meaning, in that it only applies to legal transfers of title and property, and that transferrer should be the word for other meanings. However, since transferor is much more common and probably more widely recognized by spell checkers, I am sure it is sometimes used for the non-legal meaning. I don't know how frequent this usage is.

It's transferrer/transferor. If you look at the ODO definition, it's a derivative of the word transfer.

If you search for transferer you get

No exact match found for “transferer” in British & World English

By looking at the OED definition, it can be seen that transferer is used, but only in place of transferrer and transferor:

[...] used sometimes for transferrer n., sometimes in the technical sense of transferor n.