Welcome to the forums. I know very little about Rory O'Brien frames. However, I do know that he was manager for one of Claude Butler's shops prior to opening his own shop, circa 1947. All his frames were contract manufactured. Your bicycle has a decal indicating two locations. O'Brien reportedly opened up his 2nd shop in 1956, then downsized back to a single shop circa 1974, so your frame should be 1956-1974. While the logo style has me leaning more towards the latter end of that period, the headset and lack of a derailleur hanger on the rear dropouts has me leaning towards the earlier end. Regarding the model, I suspect it is a Contessa, based on the modified Nervex Professional lugset.

While this is not O'Brien's top model, the frame is still manufactured to a high level of workmanship. The paint does not look too bad, however the stays exhibit some very heavy corrosion. For that reason, I would probably be leaning towards restoration. There is nothing so odd about these older English framesets that makes it difficult to find compatible parts, unless you want to restore it with period correct components., in which case the journey can be more time consuming and expensive, and will require purchasing from specialty sites or searching auctions sites, such as Ebay.

Links to some O'Brien information:

Rory O'Brien - cycle builders

rory-o'brien-2-builders

Link to OP's photo album:

https://www.bikeforums.net/g/user/514914