Oakeley is older.
Glodda Ganol is Welsh for Middle Quarry. It was Lord Palmerstons quarry. Above it was Hollands quarry, below it was the Welsh Slate Companies. Eventually, the land owner (Mr Oakeley) ended up working all three himself, and knocked them all into one. Well, sort of. For instance, there is very little connecting upper and middle quarry except the odd roofing shaft.
Mr Oakeley also ended up with Nyth y Cigfran quarry and Cwmorthin Quarry, and knocked them through as well. What a meglomaniac - but he did end up with the biggest slate mine in the world as a result.