Hi, YummyMummy!
I have a mini mortar and pestle that I picked up at a kitchen store and I use that to grind my oats. Takes a little elbow grease, but I like that I get differences in the texture of the oats (plus I'm too lazy to drag out the blender
). I use the old-fashioned oats, too, not the quick-cook.
Ground oats will suspend easier throughout the bar than the heavier whole oats, which have a tendency to sink to the bottom of the mold (the top of your soap). That may be what you're going for, though, and it can look very cool.
Let your base cool quite a bit before pouring if you want the oats to suspend. I pour when I feel it getting tacky and sticky.
And you can always buy an oatmeal base. I like to add my oats to a goat's milk base.
Good luck on your project!