What Pana Lens is worth considering Over Olympus lens if you use OLY body?

If you have an Olympus body, then you don't have to worry about whether a lens has OIS. So the full range of Olympus and Panasonic lenses will be stabilized on your camera.

Both Panasonic and Olympus make some really great lenses. They both also make some pretty average ones. You should just select the ones that suit your shooting style, shooting needs, and budget the best.

Aside from the kit lenses, there really are no "exactly the same" lenses. And even those have some variations.

In every single case you sited, something is different. Like your comparison between a 12-50mm weather sealed lens to a 14-42mm non weather sealed one. Or comparing faster lenses to slower ones, or comparing a 17mm lens to a 15mm lens. They really aren't the same thing.

The closest "same lens" I can find is the Olympus 14-42mm EZ compact lens and the Panasonic 14-42mm PZ compact lens. And BOTH sell for around $200 on Amazon. And both are considered fairly mediocre lenses. The Panasonic lens has the advantage of having OIS, which is of absolutely no benefit to the Olympus user.

But even if you don't need OIS, it never hurts to have it. Because you might someday own a Panasonic camera, or there might be a dual IS system developed for Olympus cameras. So I don't see it as a negative.

If the Panasonic lenses cost more it isn't primarily due to OIS. It is due to them being wider, faster, or branded as Leicas.

It really will be interesting to see some tests on the new Panasonic 42.5 f/1.7 and the soon to arrive 25mm f/1.7. While the Olympus versions are excellent, these might be even better, thereby justifying a higher price.

We have a problem that some other brand users wish they had. Too many good lenses to pick from! So we quibble about their prices and try to find things to make them seem better or worse.