I'm a newly qualified diver, so my experience is limited. However, I think unless you have difficulty with some element of the OW course, then most people will pass it first time. That said, one person in our class of four dropped out as it wasn't for them, another geared up for his first OW qual dive, got in the water, but wouldn't go down, as he was not comfortable in a salt water environment, so will be doing his qual dives in freshwater this weekend.
Visibility was less than 2' on my first two dives with strong currents, the instructor had to hold on to our BC's when doing drills to stop us drifting away. I was decidely uncomfortable. My next dive was cancelled due to low vis, less than 12", it was almost black at 25' and strong currents. Strangely enough I actually enjoyed the experience. I finished my qual dives in a Lake with 6' vis and thought it was great.
I have no worries about going out in to the ocean, but still do have some minor concerns about my buoyancy control skills, something I am working on improving and would have liked more practice on. I have completed the course work and qual dives for ANDI CSU and AOW, but can't be AOW certified until I get more dives under my belt (SSI requires 24 OW dives). While on paper I may have the requisite skills for AOW, I believe that experience is the key and something I need to accumulate.
I think Virgil sums it up very well with the car driving analogy, experience allows you to build on what you were taught and further develop your skills. However, you should only dive within your confidence level.
If you are not comfortable diving in the ocean or anywhere else for that matter, then simply don't do it until you are ready. In my limited experience I've already seen people do some strange things, because they were not comfortable with the environment they were in. Potentially, not only putting themselves at risk, but also their buddy.