You probably can take an ox to Oxford, there's horses there so I don't see why oxen would be ruled out. What you probably can't do is get an ox through the traffic on Abingdon Road if you're taking it to work.
Nothing suggests it would have been free — in fact, if I owned a ford (a shallow crossing point) running through my property, you can bet I would charge for it.
... but you can't make him drink, is what I thought you were going to say.