Let's look at the current model of a buy-side OMS connecting to an incumbent execution venue. Take your pick, they all work in a similar manner...
So we see a buy-side OMS connecting to the Execution Venue deployed UX which then connects to the Execution Venue and hence to the banks with which you trade.
Realistically this will not be an accurate representation of the world of a sophisticated buy-side - instead it's more likely to look like this:
Update 27th October 2018
End of the line for buy-side OMS
Summary of whitepaper on the evolution of the multi-asset-class trading desk
Whitepaper on the evolution of the multi-asset-class trading desk
Driving OMS reform