First the PCl5 dissociates on its own into PCl4+ and Cl-. This provides an attacking site for the lone pairs on oxygen in ROH, as well as produces Cl- ions. The oxygen in ROH behaves a nucleophile, and attacks the slightly positively charged phosphorus, donating one of its electron pairs to form a co-ordinate bond with it, to form RO(+)H-PCl4. The chloride ion then picks up the alkyl group(R) to form RCl and HOPCl4. The HOPCl4 then somehow becomes POCl3 and HCl. This is my take on it, but I am not sure.