The easy one: OSL: Arriving on QR business and connecting to AA economy (2 hours is a bit short, but hey ho) - yes you can use your inbound boarding pass just save it as the lack of ticket linking would mean you need this (there isn't a need to be on the same ticket to get "same day access". Though conceptually after all this business class flying I am surprised you don't have status now? I've no idea on OSL lounges though, that's far from home.

ICN - there is only one oneworld lounge, opposite the Korean Air one. It isn't anything to write home about, but does have a good view of the planes, and if you like hard liquors that will work out well. Food is poor and sparkling wine was from memory from somewhere random and not very nice
MNL - I've not been for a long time, I suspect the CX one is the only nice one, but is going to start to feel a bit "samey" as other outstation lounges (Cathay is consistent, the only variance is the quality of the champagne/sparkling wine, plus minor local food specialities to add to noodles and dim sum). Depending on the time of day you're coming thru there is JAL too (mornings), I've never been tempted to try
KUL - Well, its really just the Malaysian airport lounges. A shame you aren't oneworld Emerald by now, as the first class lounge is excellent with table dining. Don't waste time going back to the domestic terminal, it really isn't worth it - the lounge is much more basic, plus immigration + security, not a good plan.

In both cases above, with MNL and KUL the recommendation is to skip the contract lounges used by MH/QR respectively as they are only very rarely anywhere near as good as a proper airline lounge (that is run less like a contract centre). You could pop by at the end to see what you didn't miss out. Enjoy anyway!