Now if there is one thing that makes me feel like an old geezer it's OW certification. I did mine in '83, it took a week of my vacation. we needed 15 dives to get the little card (actually a passport-like booklet with 4 pages
). These days, it's a bit of pool and class time and if I remember right 4 dives. Around here in SoCal that means 2 each from the beach and a boat. We can probably all agree that what you get is a learner's permit at best ...
So xCR training is much better, much more in depth. Just about any course I know of recaps EANx diving anyway, and most instructors let you do both cards in one course. I'm wondering, how hard can it be to suck on a second stage?
The important thing, 'keep breathing', is the same. As long as you know your tank's gonna be empty a lot sooner OC, than xC you have a lot already figured out.
So OW on an SCR shouldn't be a problem at all. Which is probably why it has been done already, as mentioned somewhere in this thread. I've got that German mag somewhere in my boxes (just moved), it was an intresting idea and worked. The two 'test pilots' liked their Drägers much better than OC.
I think it's comparable with getting your OW in a drysuit. Some places call it advanced, want your to take your OW and AOW first, others do it without thinking twice. It isn't really that big of a deal, and neither are SCRs. A bit more pre- and post dive preperation with a lot more importance, a few more things to learn and do UW, like checking loop integrity or a flush. I'm sure most students could do it just fine, especially if they have a week to learn it all.
CCRs on the other hand are quite a bit more involved, more data to digest and actions to take. I can see how task loading can be too much for a novice on a CCR. Mixing manually while monitoring the gauges, worrying about your bouyancy and being afraid of the upcoming mask clearing ... maybe not the greatest idea. I believe any diving that entails hypoxic mixes should be far enough down the line that students can give the breathing mix their full attention, rather than worry about buyoncy, ascent rate, or just being UW. And yes, I'm aware that the SCR can be overbreathed to hypoxic levels, and that new divers frequently, if not always have a high breathing rate. But on an SCR that's easier managable than the manual mixing on a CCR.