I do this every single week - make my crisp toppings ahead, bag them, toss them in the fridge. And I make a topping that's got substantially more grain in it than you're talking about. No sogginess, perfect results every time, and so much more convenient than having to make it right then.
This was made with a topping that had been in the fridge for more than 14 days. It was perfectly crisped and lightly crunchy. And this is over peaches, which are notorious for sogging out the topping.
As Chris Hennes points out - even if the flour and grains take up moisture from the butter, at the end of the week you're going to bake it. At that point, ciao extra moisture.
