So many choices as you can see. Our current favourites are QR from OSL or AY from HEL. Oslo has some great museums for an extra day on the way;,Helsinki has salmon soup and saunas! However if you make a transit stop in Jan you will be carrying heavy coats to Oz -packing for both climates is an extra complexity
Best of all is to take a Round The World fare. The,base business fare on oneworld (“DONE4”) starting in OSL is 40% lower than a straight LHR-SYD-LHR business class fare.
Indeed, I was looking into a US-Australia roundtrip in J and found it wasn't much more to book a RTW from OSL and replace a award flight from Europe to US with the beginning of that RTW ticket.