Is it possible to take a river cruise from London to Oxford? Or better how far up the Thames can one reach on a commercial cruiser?
The River Thames is technically navigable as far as Lechlade, a few miles upstream from Oxford. However the river narrows significantly just above Oxford and is not used, as far as I know, by cruise boats. Boat cruises are available from Oxford going downstream - this Google search will give you a start point. This page and website will give you links to other cruises around London, and places between the two.
As far as I know there are no cruises that run to Oxford from London (or vice versa), because of the distance and the need to navigate multiple locks. Going one way would occupy at least a day's sailing at riverboat speeds, and there isn't much demand for multi-day cruises.
In order to see as much of the Thames as possible, I would recommend multiple cruises starting from different places. Boats operate out of Oxford, Windsor, Henley, Caversham (near Reading) and Maidenhead, as well as London, and there may be other places.
If cost is no object you can organize your own private boat cruise (see the River Thames site above). For the right price they will probably go as far as you want, but without a significant number of people the cost is going to be extreme.
As a much cheaper alternative to private charters, rent a self-drive boat for a week. These are surprisingly affordable, and need no qualifications to drive. A week rental will allow you to see a large chunk of the Thames. The rentals are generally in the Upper Thames (i.e. outside of London). You won't be allowed into the busy commercial waterway that is the London Thames, but outside of that you are free to go where you want. Hoseasons is the market leader, but there are others as well.