Before you get to the Thames, fill up with diesel at Aynho, that will be the last you see at anything like a normal prices for a long time.
Personally it find it more pleasant to get onto the Thames from Duke Cut, the canal down into Oxford is a bit grim and very slow going.
Go all the way up to Lechlade first, there are lots of nice free end of field moorings all the way up, the only place you need to pay is at Lechlade and that is only £5 (I think) a night. Well worth chilling there for a few days if you have time.
For mooring in Oxford you can stop on the already mentioned East Street moorings above Onsey lock. There are EA moorings and are free for the first 24 hours and then £10 a night for max 2 additional nights.
Abingdon is great with lots of good free council mooring for a max of 3 nights.
As you get further down there are fewer free mooring, but there are still a few EA 24 hour mooring, usually close to locks, but expect to be paying mostly £12 a night to moor from now on.
Reading has not go too much going for it, except that Caversham Boat Services and Better Boating in Reading are the only 2 places I know if the sell diesel are normalish prices and allow any split.
Henley at a weekend is mad, but it is good to moor on the fields downstream of the bridge and sit is the sun and watch the mayhem. Don’t think about moving!
At Windsor you get great views mooring on the Eaton side but the bank is very high for a narrowboat, and if levels are low it hard to get into the side. The Windsor side is earlier to moor and is the same cost, so you pick which you like!