I don't believe all the creative team and Jyp himself would have believed that O.O sounds good.
But debuting with it worked well for nmixx. They were overshadowed by their senior groups Twice and Itzy. But the debut made them memorable as a group with worst debut song. And the members got praised a lot for their vocals and dancing. Now everyone will be curious for their first comeback if it will be similar to o.o.