Hmm, I'm not sure about that. I
am sure that you're wrong about there beings OT rounds as late as UFC 28. They were already using the 3x5 and 5x5 round system by UFC 23 when Randleman captured the vacant belt in a 5-round decision victory over Pete Williams. Frank Shamrock also put Tito away in the fourth round of their five-round championship fight at UFC 22. When Jeff Blatnick started overhauling the rules as the UFC "Commissioner," I know that they tried different things out event to event, but actual OT rounds? Even if this era isn't relevant to your question, I'm curious now to know when exactly they stopped the regulation round(s) followed by OT round(s) system.
And my old school knowledge is rendered useless. I will await the answer from someone up on the new school