ISSUE:

Good evening,
in the witing reaction, once the 4-membered ring is formed between the O, P and the 2 Cs, why does the O form the double bond with the P?


What force drives you to it? All the best

The P=O bond, with its bond energy of 575 kJ mol–1, is one of the strongest double bonds in chemistry, and the Wittig reaction
is irreversible and is driven forward by the formation of this P=O bond. (Clayden)

Clayden? I suppose Jonathan Clayden? Is there more info about the bond P=O?

Yes, it is from Clayden's book, but if I remember correctly, there are no more details about that aspect in terms of the chemistry of P.
Greetings