I prefer the taste, it gets a tangy, sourdough-like flavor. 

We generally soak around 3 cups of oats at least overnight and up to 3 days (I'm sure you could go longer) with a 1/2 tsp(ish) of salt, 1 Tbsp(ish) of kefir, and enough water to cover. 

Soaking starts fermentation that neutralizes phytates that prevent digestion and bind to the calcium in oats so you can't assimilate it. The phytates are what give lots of people gas after they eat oats. Soaking also reduces cooking time somewhat for plain rolled oats, presumably for groats as well.