I don't necessarily think that iPhone takes presidence over the iPad in terms of OLED. But if they do release an iPad with OLED in terms of hardware, as it's been rumored, it would make sense why the OLED iPad would release a little later due to parts supplier constraints. Either way, it doesn't make a difference to me as long as we can see something separating itself from the other iPads with this new 10.5 inch form factor.
The iPhone is Apple's biggest selling product and produces much more revenue for them, new technology is often put into the iPhone before it is later brought over to the iPad (Touch ID for example). I'm interested in the 10.5" iPad Pro but being a 12.9" iPad Pro user I'm not sure if the amount of screen real estate that i will lose would be negligible or not. Guess we wont have long to find out.