Transitive verb and Godan verb
おいこす
他の自転車走者を追い越す時、いつも挨拶代わりに手を振ります。
I always wave my hand to say hi whenever I pass other bikers.
越す means "to go beyond." 追 is follow. If you are following someone, and then you go beyond them, you have overtaken them. That's why this word means to overtake.
The reading comes from 追う and 越す put together into one. You can overtake this word now that you know that.