I just found the OP to be a little more dressy and liked the steel crown on the dial too. A little detail is also the shape of the bezel: flat for the Explo and convex for the OP.
Since 1926, OP was created and offsprings like Datejust, Submariner and Explorer were created. The pure and quintessential of Rolex which many have been swayed because of hype towards steel sports watches. ...