I am posting this far too late to be of use to anyone else (closing time was hours ago) but I enjoyed a free oyster today at Haymans. They have had a branch in the Oxford covered market for a long time but, last year, opened up another in the Osney Mead industrial estate on the edge of Oxford. Unlike the covered market, the out of town one enjoys easy, free parking. I first visited it a couple of weeks ago and decided to pop back today. As well as coming away with sea bass fillets, whitebait and venison (the latter is one of their sidelines, not some strange aquatic version), I had my free promotional oyster and a little sample of wine.
Although Oxford is about as far from the sea as you can get in the UK, it has now become much easier to regularly add good quality fish to my diet.