From the Anandtech article[1] on the M1, I think this is referring to the reorder buffer, or ROB. Reorder buffers allow instructions to execute out-of-order, so maybe that's where the "OO" comes from.
Yes, I was going by memory, which thought “out of order buffer” was a thing. I meant ROB. Point still stands, in that scaling these blocks is very difficult with variable instruction size.
