That's not really true, Norway was one of the richest countries in the world long before the oil boom. In Europe it was only behind Luxembourg, Switzerland and Sweden by GDP per capita in 1960.
You're right. I've lived in Norway and was repeatedly told by Norwegians the country used to be poor, something I believed (there isn't much in Norway that reminds of a wealthy past). Apparently it's just a myth [1][2].
[1] https://www.dnva.no/detskjer/2019/11/en-feiloppfatning-norge...
[2] https://www.aftenposten.no/meninger/kronikk/i/LM1L1/Hvorfor-...