Oslo pass quite ex isit really worrh it?
Really depends on your taste. I felt the museums in Oslo were nothing great compared to those in other major European cities. Maybe with the exception of the Viking Ship Museum. And then there Munch's "The Scream" in the National Galllery, but the rest of the collection is pretty average. There are other museums dedicated to Munch!
If you're not into museums, you might be better off just buying a 24 or 48 hour travel pass that gives you unlimited free transport in Oslo (except the Airport transfer). The Oslo Pass includes the ferry to Bygdoy peninsula, which the regular travel pass doesn't. The travel pass can be bought with the public transport app, but I guess you can get it at the airport/train station too.
What to do in Oslo for 2 days? Apart from the museums, you can walk along the waterfront, where the opera house is. There's also a ski museum (Norwegians appear to be born with skis attached to their feet!!) and a sculpture park.
The walk along the river Akersvela is nice, and it ends at a big food hall, Mathallen, where you can eat relatively cheaply (by Norwegian standards, but not the rest of the world's). The Grunerlokka neighborhood is pretty hip.
I felt completely safe walking around with my kids, but do note that the area around the Central Station looks a bit dodgy. Quite a number of people high on drugs even during the day. Good exposure for sheltered Singaporean kids!