Re: Adding oats to egg white liquid/powder
The only thing to worry about would be bacterial/fungal growth in the oats, and those won't grow very quickly in the fridge. With that said, I think morning to afternoon is about as long as I would go unless you've boiled the oats.
Another option would be to grind the oats into a powder and add it to the drink just before you drink it. That somewhat defeats the purpose of slow carbs though.