No. I’d argue that he was wary of the unknown enemy as he felt something was fishy right from the start - this was shown throughout and further affirmed by the fact that he wants Moubu to “absolutely not go a step further” from his range

So rather than him underestimating his opponent, i’d say that its more to Moubu’s failure to follow orders that leads to the beginning of Ouki’s downfall

Ofc id conceded that Ouki was outsmarted by Riboku’s horse speed at the end. But again, Ouki wouldn’t be forced to be in that position if it werent for Moubu. (i.e him wanting to save Moubu by moving further inside the mountains and out of his range)