How much carry-over the diamond push-up has to the one arm push-up depends on the variation of the OAPU you are aiming for. In Convict Conditioning, they are a progression, however, their demonstrated version of the OAPU is highly unusual and could be thought of as a one-arm diamond push-up. In most cases, people will get a wider "Rocky" type of OAPU first, which has very little in common with a diamond push-up and much more with an archer push-up. Again, there are different types of archer push-ups, just like there are different variations of the one-arm push-up; apart from how far the elbow is flared out and whether the hand is placed under the chest or further out to the front, one main difference is whether one looks towards the pushing hand or away from it. Another possible variation would be gripping the wrist of the pushing arm with your other hand to assist; this may be seen as a progression from the diamond push-up, however it allows for bigger freedom in hand placement. And of course, there is partials, incline OAPU etc.
But back to the original question: rep ranges with progressions can be somewhat subjective, and also depend on the progression used. That is to say: someone who responds well to higher reps will find it beneficial to work more in that range (say, 20-40), while someone doing better with higher tension may get enough stimulus from 5-10. Also, the bigger the jumps in difficulty in the progression, the more work (in terms of intensity and volume) will be required for each progression.
So in short, it depends. I would suggest deciding on what variation of the OAPU to work on first, and to look at what rep ranges have tended to work for you in progressions in the past. Then tweak and adjust from there.