it's little strange for me Polish but I see it like this becuase in English you don't have Our the oven so it looks more like: The oven of ours is small?
In English it is Our oven is small..although in a lot of cases with "ul" on the end it means "the" before the noun, in some cases it doesn't..I know, it's confusing ;)
Yes, it would be something like "the oven of ours is small". That also happens in greek, in italian with a different look, and you can use it in spanish and other languages.
Why not: cuptorul nostru este mic