You're limited to four flights within the Europe/Middle East "continent," and two stopovers.

Because there are no Oneworld longhaul flights that operate from Oslo to other continents, your first segment will have to be one inside the region - to some Oneworld destination like Doha, London, Helsinki, Madrid, etc. Likewise your last flight will need to get you back to Oslo, with the same problem but in reverse.

So on my imaginary trip, I'd have you returning to Edinburgh from Japan via Helsinki. That would be your second Europe segment, the first one having been OSL-LHR at the beginning. That would put you in Edinburgh with two of your four flights used, and one of the two allowed stopovers. At present, as far as I can tell (and I'd be happy to be proved wrong) the only airports in Europe/Middle East with direct service to/from Edinburgh are a few in the UK, Madrid, Helsinki and Doha. Everything else would require burning a segment from EDI to the likes of LHR, leaving only one Europe/ME segment remaining, which would have to be to Norway. Hope that makes sense, thus my suggestion is that your Europe/ME segments and stopovers be OSL-LHR, HEL-EDI, EDI-DOH, and DOH-OSL, with stopovers at EDI and DOH. Now you could swap HND-HEL with HND-LHR and use London as your first stopover (head home to EDI on your own) then go someplace else from London on the "bonus" holiday, but you'd still need to reserve a final segment to get to Oslo, with the same limitations on airports with direct flights to OSL that you had before.

You might also want to consider the impact of your routing on your BA status. Finnair is moving to an Avios system (Qatar and Iberia already have) but I don't know how flights operated by AY or QR count toward BA status or if they satisfy the "4 BA/Iberia metal" requirements to advance in BAEC status (in addition to tier point thresholds.) This is something you could/ought to review in light of any other travel plans you might have for the year.