The latter is responsible for slotting himself into downwind traffic, and likewise ATC can't assist him with that.
AFAIK the latter is responsible for giving way to all aircraft in the circuit, including the one taking off.
In any case, the way I see it is that the OHJ creates no less than three areas of conflict where you might encounter traffic on a 90 degree or more collision course. First in the overhead itself, then when crossing upwind, and then joining downwind. Any other kind of join only has one - the place where you join the circuit. And considering that the downwind leg is the longest circuit leg, thus giving the best opportunity to spot other traffic and slotting in neatly behind, I know what sort of join I prefer...