Great choice if you don´t have much (or any) money. Meals usually cost about 50 czk each!!!
Not every food is vegan, but there are usually some possibilities. Oaza is more bistro than restaurant (no chairs, only counters where people eat). It isn´t a high class cuisine, but it´s kind of place, where you eat (or take away the food) when you hurry. Food is quite healthy (some veg protein + wholemeal side-dish, buckwheat, millet...personally I hate it, but for this price? :D) and I think that it is really tasty as well.
You need to try it on your own, but I think, that there is no cheaper bistro in Brno (and all Czech Republic as well?) and I appreciate the equality of price, quality and quantity.
Cons: No possibility to sit and eat, common interior