Have you ever studied a language for a ton of time and then as soon as someone says something to you in that language you freeze up?
If you don't go into that situation warm, of course you're going to freeze up. And no amount of grammar, vocabulary, or flashcards is going to help you.
Real life conversation is the final boss of language learning.
If you go in cool, or even normal temperature, you're going to freeze up. You have to go in already warm.
The purpose of OPI training is to get you warm.