You are at the happiness cap, if a further grow of the city would result in an unhappy citizen who doesn't work.
The difference between Oslo and Stockholm in your game is probably because Oslo is the capital and gets culture from the palace. Unlike in vanilla civ 4, in this mod happiness ressources don't affect every city of your empire, but are distributed among your cities in order of the city-culture. Oslo as most cultured city gets happiness ressources first. You should build something that provides culture in Stockholm, so Stockholm is at least no. 2 among your cities in culture, and not small cities like Bergen, wich don't need ressources to stay happy anyway. You don't need tons of culture in Stockholm, just more than your other cities.