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Just from the top of my head you have Google Flights, KAYAK, SkyScanner, Expedia, Momondo and Hopper. Although there's probably a ton more of this kind of flights aggregator websites.
I personally use mostly Google Flights and KAYAK but you should try them to see which one fits you best.
EDIT: Just made a querie for return flights from Oslo airports to destinations across europe for 1 week during the next 6 months and found plenty of cheap flights.