The lounges at OSL are in the Schengen area (i.e. before passport control when leaving) so you’ll have no issue. Also note that the connections channel was closed when I passed through in early September and so you had to go landside and re-clear security when connecting from non-Schengen to Schengen. It’s been that way since the pandemic and I don’t get the impression it’s changing. I suspect part of the reason is because on-arrival duty-free is massive in Norway and you would otherwise by-pass this going through the connections channel.