If a 1999 voyager smokes when sitting in traffic and has started leaking oil what could be the problem?

Those are two different problems...

If it's smoking in traffic, it could be from many things...

If the smoke is coming from under the hood, you might have an oil leak that is dripping onto the exhaust.

If it's smoking from the tailpipe, odds are, your rings / compression is gone and the oil is blowing by the rings and getting burned.