BA aren't recruiting at the moment and I doubt they will for a long time. Latest news says they are trying to cut down their staff and aircraft. Future recruitment will probably be experienced pilots only because of the huge amounts of them on the market.
If you go to OAA you will be required to do their PPL course again and will cost in the region of £70,000-80,000 for the full course plus a secured bank loan with interest so you're talking well over £100,000.
If I were you, I would do your training at a decent FTO for a decent price, join a good airline and build up on the hours. Once you have the hours and experience I would then consider joining BA if they are looking for people.