In every offer letter I've received so far (not Oxbridge unfortunately, or at least not yet ) the wording is "we (the departement/college/admissions team) are to recommend that the University makes you the following offer"
They all do make the offer on the letter, but I found it a bit weird that all 4 letters were phrased in this way. So it probably is so for a reason - to stop it becoming legally binding.
I can only imagine that Oxbridge letters will say the same.
Of course, once it's on UCAS it's binding.
Has yours come through yet?