My experience is that the melees (usually including me) average about 1 OA/encounter, barring Feats and weird situations and so on, in 5E. Some specific situations can massively boost it - i.e. to practically 1/round, but it's rare. More common is an encounter where no OAs happen.
This is a stark contrast to 3E and 4E, where spells and ranged attacks provoked OAs, as did a bunch of other stuff, and where you could have multiple OAs very easily, so many rounds had loads of them.