O&D is Origin and Destination ie non hub traffic.

In terms of another airport I think thats the general idea of the reasoning behind expanding Stansted with its new runway well before Heathrow. However Stansted is a long way behind Amsterdam, Paris, Frankfurt, and others in terms of offering a strong O&D market as well as hub capabilities. In my opinion by the time Stansted is at a similar stage to these airports, the hub traffic will have long moved onto the continent and sapped away the competitive advantage of the UK travel market.

Hopefully rdc1000 can elaborate on anything I’ve missed out or got incorrect as he is far more of an expert than me on this kind of thing.