There is some comment in the Dashboard's Europe lounge thread about this issue. This is new route, and typically they don't give lounge access for the first year or so, the wiki on the previous mentioned thread has a long list of such locatons. Admittedly many of BA's new routes are to locations where lounge provision is not going to happen, the airport is barely a barn at the end of a field. OPO however would have some business traffic and has been going for a while. At some point lounge access was granted at the ANA lounge, I used it at least twice, and yet more recently it appears to have been taken away. I don't know the background, but typically this happens when the lounge operator bumps the price beyond BA's limits (e.g. PFO).

Which is a long paragraph saying that lounge access seems unlikely, but it would be good to get your experience of this. You would get lounge access in LGW, albeit No. 1 Traveller.