I think that the rivalry is between Helsinki/Tampere vs. Turku, i dont think that there is bad blood between Turku-Oulu.
Personally I think that both cities are beautiful, especially in summertime, when there is miniskirt season and daylight almost 24/7.
Turku is a beautiful city that is much more fun to live than Oulu or other citys in middle of Finland. Only problem is the people living there as their accent is terrifying and that the river smells like ass.