An oven is a process to cook food. It functions exactly like a furnace. However speed depends on tier. 

Ovens(like spits) can only cook food. anything that is not edible will not work.

OVEN TIERS:
Bronze (half the speed of MC Furnaces)
IRON (same speed as MC Furnaces)
STEEL (50% faster than MC Furnaces)
MITHRIL (2x faster than MC Furnaces)