In reply to elliptic:
Actually, I don't think that diagram is entirely correct. The area that is on the move extends at least to the line of the upper part of OB in my opinion (and that was formed after inspecting the whole area on a rope). I think that the entire top block is hinging more or less from the blue number one at the top of the line of OB downwards to join the hatched area but extends to the right of the line of the last pitch of OB in places and may well lie down the crack at the top of OB's ramp rather than the one behind the yew tree belay further left. If one imagines the whole top block as a huge door leaning against a wall then the left-hand top corner is both opening out and falling forward at the same time, while the bottom right corner is staying more or less stationary. This is why there is an ever widening crack at the top left and a number of loose flakes on the right (rather than a crack).
In response to the question, “Is it safe to climb OB?” the answer surely has to be not nearly as safe as it used to be (and quite possibly seriously dangerous - who can tell?).
Quite honestly I wouldn't want to be anywhere North Crag if this lot actually fell off in one go and wouldn't climb anything where there was a chance of being hit by falling rock from this area.