"I want to spend more time in Oslo."
Translation:Haluan viettää enemmän aikaa Oslossa.
Lisää is more, additionally. Enemmän is also more, but it's the comparative form of paljon, a lot. http://randomfinnishlesson.blogspot.com/2016/09/lisaa-enemman.html?m=1
Now that you put it that way, yes. You can say only "enemmän aikaa" when you're planning your first trip to Oslo, but you can use also "lisää aikaa" when you have already been there for some time. It would sound especially good when the end of your time in Oslo is getting close, but "enemmän" is always an option here.
Thank you for this additional information. It's always interesting to learn different aspects of the language.
But in this particular case I didn't mean integrating haluankin in the whole sentence, but as an answer to it, just referring to CthvlhvsMagnvs's comment. I understand his "haluankin" as an attempt to say "So do I" twron in Finnish. And I think that the "so do I" construction would be verbkin - haluankin in this case. (Trying) to make it clear:
-Haluan viettää enemmän aikaa Oslossa.
Please correct me if I'm wrong.