I have around 15 square meters of oregano, I never watered it and it's entirely under the sun, all the day. Unless I'm lucky (I'm kidding here, it's science, not luck), I'd say oregano doesn't need any special treats to grow, especially if it's in his natural environment like in my case.
Adding more to this, I remember going to the countryside with my mom when I was younger to harvest some oregano, and it was in an abandoned land as well, which makes me think it really doesn't need extra water to grow, assuming it's in his habitat of course.
This said, I'm pretty sure it would still grow better if particular care is given, which of course includes watering it from time to time, but from my experience, I wouldn't say it's particularly hard to grow it, all it needs is sun light. On the other side, beware too much water could damage its roots which could become rotten, making the plant die, so make sure there's no water stagnation and you'll be fine.
And as an extra, here are some pictures of my oregano (click on the pictures to enlarge them):
We're over flowering period or you'd see them completely white, and we already harvested A LOT of it, but it's really so much that we never run out of oregano. Consider it didn't rain for over two months here, and oregano is still completely fine and healthy as you can see.
I hope it helps!