Try doing it from tables when you are just being exposed to Nitrox. SDI still teaches OW using tables. Trying to keep the two straight while figuring out multiple dives with varying SI's and some gotcha's thrown in - just to make sure you understand. Yeah. Thats too complicated to throw at a noob. They have enough trouble trying to master the physical skills. They don't need additional mental stuff thrown in.
OW card is just a learners permit. No additional task loading should be added until the critical skills of OW have been mastered. If, as has been posted above, people woking at a facility sometimes fill a nitrox tank with air, an air tank with nitrox, and don't get the gas mixes correct, what do you think is going to happen to someone who got nitrox on the fly during OW? Hell the training has gotten superficial enough and you want to add more material to be breezed through. Bad idea!
Interestingly, we've been teaching Nitrox as part of the entry course for a number of years (British Sub Aqua Club).
At Ocean Diver (Entry) you learn about Nitrogen, Oxygen, Air, Nitrox, and managing Nitrogen on an Air table (and Computer). You also learn that any ascent after a dive is about controlling the Nitrogen (controlled decompression).
At this level they only have the one table to learn.
At Sports Diver (a cross of AOW and Rescue Diver). This is the point that you learn to use a Nitrox tables, and other 'Nitrox Issues' (you will qualify with a 35m (115ft) decompression ticket). This is where you also learn about staged decompression.
Interestingly, Its not really been an issue for the students.
In simple terms they learn that are a number of gases you could breath. they have pros's and con's, which are covered in more depth in later qualifications (The third level up introduces the concept although not the use of Trimix).
One thing that has been noticeable, and was really the same when it was only air - but no stop diving at Ocean Diver, and staged decompression at Sports Diver. Is that students do really need the reinforcement of the second qualification when it comes to tables, decompression theory and Nitrogen.
That said, its the same with the PADI cross overs we get. There is a significant lack of understanding about tables and decompression going into the Sports Diver course. Then, a noticeable better understanding after they finish.
One other point to note. Although Ocean Diver is a full diving qualification - basically PADI OW. Clubs ensure members at least progress through Sports Diver. Most clubs don't regard the Ocean Diver qualification as a sufficient level for club diving. Ocean divers are managed by the club, normally diving with very experienced Sports Divers, Dive Leaders (DM), or Instructors. Under controlled environments, Ocean Divers are encouraged to dive together.