According to signage, this route ends at US 259. However, most maps and ODOT documents have it continuing along 259 to end at US 70 bypass. This is likely a relic of the time before US 259 was commissioned circa 1963, when the entire highway was OK 87 and the Red River bridge didn't exist.
Control section maps show it as ending at 259, same as signage. So I'm inclined to believe that is its "official" terminus. Otherwise, your research matches mine.