Frist, Kolonihagen is quite nicely placed at the back of a garden ("hage" means garden in Norwegian). It may surprise you a bit at first but it is quite cute.
The buffet has both warm dishes and cold dishes. Everything is home made with organic products.
The buffet generally includes some meat dishes, vegetables and a savory pie.
Cold dishes include starters, cold main and desserts. For example, organic bread, salmon or trout, tomat and mozzarella salad, etc...
Example of a dessert: apple crumble pie
It is not relevant to give so much more details about what we had as it changes every time and you may have something completely else than what we did. However, the dishes taste good, the choice is quite nice and overall, this is a nice Sunday brunch.
If I recall correctly, the buffet costs about 300 NOK for adults and 150 NOK for children (from 3 yo). The official website (only in Norwegian): http://kolonihagenfrogner.no/. It can get full quite quickly so it is a good idea to book a table in advance or to avoid busy hours (go there early for example).