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OSL-xHEL-BKK-sHAN-NRT-HAN-sKUL-MNL-SYD-sBNE-xSYD-AKL-LAX-sLAS-MIA-xLHR-OSL

Your itinerary is valid, but could perhaps be improved

Lingo Lesson: not -sXXX rather ,XXX for a surface segment

OSL-xHEL-BKK
AY has a daily flight HEL-BKK which is an overnight flight
Maybe instead OSL-xDOH-BKK flying QR. It has a daylight 787 flight OSL-DOH and a number of daily flights DOH-BKK
This would however mean you cannot use the online tool to book (it doesn't handle QR as the first carrier)

S.E. Asia
You are being very wasteful here
Using 5 segments BKK,HAN-NRT-HAN,KUL-MNL
Better would be 3 segments
BKK,KUL-NRT-MNL

Australasia
Why BNE-xSYD-AKL
why not BNE-AKL

North America
Depending on the time you have for your road trip LAX-LAS, maybe return to LAX (we did such a road trip 4 years ago - drove from LA to Las Vegas, and on to the Grand Canyon, then up the central valley to San Francisco and down the magnificent coast road back to LA)

Europe
MIA-xLHR-OSL
You are London-based, so why not stopover there and some time later (up to 12 months after the first flight) finish off by returning to OSL
You are allowed two stopovers in your continent of origin
If you have a spare segment then you could fly LHR-XXX-OSL