Yes, indeed roasting is one of the factors that has a concrete effect on caffeine content.
Just as with the houjicha, oolong tea that is roasted or baked will also see a reduction in caffeine. When tea is roasted it gives off a bitter, white powder (guess what it is!) Mr. Kaiser from Rishi shared this photo of him in a tie guan yin roasting room.
When aged oolong is repeatedly baked periodically, it leaves behind more caffeine with each baking. So, if you assume all else equal between two like teas, the more roasted can be safely assumed to have less caffeine.