I think your Mr. Opuntia is what is usually called Opuntia vulgaris locally.
Looking into it this plant's valid name actually appears to be O. monacantha, with the name O.vulgaris actually being a synonym of Opuntia ficus-indica.
Still, people seem to be inclined to use the familiar, if incorrect name of O.vulgaris, probably because they don't know it's incorrect lol.
It will definitely grow outside here, in fact it's practically unkillable.
Indeed I've seen this plant naturalise to some extent in outer suburban areas and especially along Mel trainlines. See it from time to time in gardens, both cacti-orientated and general. It also makes regular appearances at weekend markets.
It does not get anywhere near as large as O.ficus-indica.
You have the normal form, this one also comes in a variegated and monstrose form.