Oh yea! You totally could do oatmeal, yogurt, peanut butter, grapes, and apple! Personally, I'd prefer to cook the oatmeal first then let it cool off completely before adding to smoothie. You may or may not need water if you're adding cooked oatmeal to the smoothie. If it isn't sweet enough, you could try adding grapes to the smoothie. Perhaps some cinnamon as well if you have it, like it and are not allergic.
I forgot to add to the previous post that for more sweetness, I've used raisins, dates and figs. Usually 1/2 serving's worth or less is enough.
As for you not having Greek yogurt, no biggie! Yogurt is yogurt.