I live in Portugal and rent out my Oslo flat — taxed in both countries?

Yes — and that's how the treaty is designed: property income is always taxable where the property sits (Norway), and Portugal taxes it again as your residence state, with a credit for the Norwegian tax so you don't pay twice in full. The same logic applies when you sell. What usually goes wrong: the income is reported in one country only — and remember that a flat kept at your own disposal (not rented at arm's length) also undermines your tax emigration entirely. Rent it properly or sell it; the in-between is the expensive zone.