I am Norwegian and live in Norway, but I frequently visit England (which is the reason I am a member of this forum). Norway and Oslo is indeed expensive (also for Norwegian), but due to the recent quite substantial weakening of the Norwegian Krone, the difference between Oslo and London is in fact not that big anymore (BGP 150-250 per hour).
Sale of sexual services is legal, while purchase is not (the fantastic "Swedish model"), so stay away from the street girls. The safest is probably to meet WGs in (their) private apartments. I do not recommend incalls or outcalls to hotels, as the hotels might be eager to call the police upon suspicion of punting.
The webpage mentioned above (realescorts.eu) is probably the best page for Norwegian WGs.
Norwegian girls are as beautiful as Swedish girls (I am of course a bit biased), but at least 50% (probably a lot more) of the girls on these pages come from outside Norway. If you want to meet beautiful Norwegian girls, you have to go clubbing.