I'll answer this one even though you pursued it almost 6 months ago. My apologies, I was not active on the site at the time due to family issues.
The Nutribullet 900 is a 900W machine with a 32 oz capacity. It is designed to make a large smooth smoothie which it does exceptionally well. That's pretty much all it does.
The Ninja Professional 1000 is a totally different machine. It is cheaper and by a different company and has a capacity that is twice the volume of the Nutribullet. It isn't designed to grind only blend. You are trying to compare machines of totally different quality, of totally different brands, radically different capacities yet of very similar power. The size and shape of the blending vessel itself has undergone a lot of design to ensure that food doesn't have to ability to escape from the design of the blades. Nutribullet put a lot of research into these things.
One thing I except that you are doing is filling the cheaper larger model to its total capacity then blending. And basically they isn't giving you the results you hope for because the oats have much more room to move away from the blades and therefore don't get ground up as smoothly.
You have 4 options here. 1 is to pre soak the oats so that they are much softer and this will aid the larger cheaper blender making a better job (use the water you soak the oats in to make the oat milk). 2 don't put in anywhere near the desire volume of water with the oats so they don't have the room to escape the blades, and dilute the end result once you've made it. You'll probably also need to run the blender for much longer, so watch that it doesn't overheat.
Option 3 is both of the above. Pre-soak the oats and use ⅓-½ the desire volume if water when blending, then dilute the end result.
Finally, option 4. Nutribullet do actually make a larger capacity jug for the Nutribullet 900. You could just try that, as you could also just try adding twice the volume of oats to water in the original jug and diluting it afterwards. It may work but listen carefully to that motor of yours and listen hard to see if it is struggling too much. If it is, it will overheat and that could be dangerous.
However the only way you'll get the desire results without soaking and by adding the quantity of water required from the get go is to buy a more expensive machine from a well known company.
I've been here, done this so to speak and giving up with the cheaper companies and investing in a great quality blender was the best thing I did. It even comes with it's own washing program that we run (usually twice) with a tad of washing up liquid in with water to ⅔rds full and it becomes self cleaning to a large extent.
You need to be looking at something with a sizeable motor for the same size (64oz) container. I'd suggest something in the region of 1700W would be about right and something that is designed to make nut butters, something that can actually grind properly and get the job done. They are not cheap, though fortunately come with 10 year warranties and are from a company that has put an awful lot into the design over the years, and knows what it is doing. But all of that comes at a price. Vitamix machines are not cheap but they do the job with amazing ease. The best thing i ever did for making my life easier in the kitchen was to buy a good quality Vitamix capable to making soup hot, of making nut butters, and of nut (or oat) milks with ease, with no pre-soaking. Mine also makes nut creams with no pre-soaking as well.
