Thank you, that is what I wanted to know, if OneWorld Status has any effect on Finnair seat selection. Looks like BA Exec status has no clear benefits when flying FinnAir, apart from perhaps some TPs/Avios. I got confused because the booking showed my BA Exec number linked against it (On FinnAir website). The booking was made via a TA.

At any rate, it still worth it for me because flying with BA would cost an extra £2,000+, so I guess small price to pay.