Actually, an olive egger is a cross of a dark brown egg layer with a blue egg layer, not just a brown egg layer. You'd need a Welsummer or Barnevelder or Penedesenca or Marans, for example, to make an olive egger. A Barred Rock won't usually get you an olive egger, though I have one BR hen who...
but, then again, it may not. It's hard to peg mixes. Sometimes, they look very little like what they really are, though if you're experienced, you can see some clues to the cross.
I've had a few "chocolate chicks" hatch out and they are usually some variety of Easter Egger or EE cross with another breed, but it's impossible to really say at this juncture. Has it got a single comb? This chick looked exactly like that and she was 1/2 Delaware and 1/2 EE.
And this hen...