I'll be interested to hear the results of the pH tests. My mash and end of sparge pH for the tart beer were both right at target (5.3ish). I didn't take a reading of the final beer though.
There's some info going around that kveik gives a little more acid compared to traditional strains. One brewery I know that brews quite a bit of kveik-fermented beer actually adds buffering salts post boil prior to fermentation in order to counteract the final acidic pH. I'm just curious if Oslo is especially susceptible to acidifying. I haven't noticed the tartness with Voss or Hornindal...although, those beers were generally hoppier so maybe the perceived acid blended better with those brews (acid complements fruitiness) compared to the german-like "lagers" I've been trying to do with Oslo. In fact, lager strains themselves typically result in an even higher final pH than ale strains.