The tool has load of bugs in it, you can safely ignore that. Just go through a travel agent or the AA RTW desk, it's about $1000 cheaper from OSL than ARN, and you can use QR as the first segment that way. If you book in the tool, whether from OSL or ARN you'll still need BA, AY, or IB as your first carrier, I guess from ARN there's RJ as well which is probably how you're getting a price but be aware they cancel the BKK flight very frequently (if it's even running again now, was cancelled for quite a while over the summer) and it's a 3rd tier airline, worse even than BA.