If mated together, you do indeed get a chance of brown and normal green egg layers. The thing about Olive Eggers, to successfully breed them - Take one and breed it back to either a dark layer or a blue layer. The blue layer gives you more of a green egg, the dark brown layer gives you a darker olive. After that you can go back to the blue layer and get the same olive from the beginning again.
Some people have indeed bred OE x OE, and you do get some olive eggs out of it, but not 100% of them are.