Well, here I am on the other side! 
I took the OPIc (C for computer) on Sunday, and honestly have the same feelings that you do, feeling really grateful that I went ahead and did the test at all because it really encourages me to keep moving forward. The results showed that my study over the years has stuck, despite long gaps of not using Japanese at all, and being very rusty currently. I scored an Intermediate-Mid.
I did not do any intensive oral study beforehand, or practice dialogue with anyone. I just reviewed grammar, vocab, etc, the very basics. Three of the questions I did not understand (or realized what they were asking afterwards, too late) and really didn’t have the ability to do much with the role play because I didn’t understand what role I was supposed to be playing, or the computer’s role for that matter.
It is important to note that OPI is rating your skill, not whether you got the question “right” or “wrong.” They are trying to rate what you know, what your actual language skill is, so producing as much language as possible within the given time can only help.
I found out last week that I actually need to pass the test at Advanced-Mid in order to pass my program, which was a bit of a surprise. But I have taken the test now and know how to prepare better, plus I rated higher as a baseline than I expected after being so rusty. I have one year to amp up my skills and pass it. I think it is very doable now. Also, after my experience with the OPIc, I am curious to see what it is like to take the OPI (with a real person). But for the purposes of my program, I will most likely take the OPIc again because I felt less anxiety at the idea of wasting someone’s time or being embarrassed at my mistakes, lol.
I paid for it because I need to know how to do better for this grad program, according to a couple advisors I talked to it’s really useful for upping your score.
