You can't offer an oath in the normal way. The only way that I've seen it work is that when they become ruler, they "already" are a noble belonging the region of the capitol.
Actually, the capital is just coincidence. The key is that they are already aligned to the region. And while that would be the capital 99% of the time, it's not always. In Perdan, my character was duchess of Castle Ubent. When she got elected as ruler, she lost the duchy and became a noble of Partora, which was the capital at the time. But then we moved the capital to Perdan, she was no longer a knight of the capital city.
Based on what I know of the code, that makes a bit more sense.
