I would not consider oats to be particularly high in phytic acid, nor would I consider the soaking of oats necessary in creating an optimally nourishing meal that contained oats. Following are some details that may help to more fully explain my position.
Raw, uncooked and unprocessed whole oats contain an amount of phytic acid that is similar to the amount found in other grains like wheat, corn, and barley. Like those grains, oats contain their phytic acid in their outside layers. For this reason, whether you are making oatmeal, or oat muffins, or our Swiss Breakfast recipe, or just eating a pre-packaged rolled oat cereal or bread made from oat flour, the level of phytates in your oats is going to have been naturally reduced as a result of cooking the oats. This reduction seems natural to me and I consider it a part of healthy eating.
One of the long-standing concerns about phytates in food has been their ability to interfere with mineral absorption. When oats have been cooked, or milled into flour, their phytate content will typically fall into a range of approximately 2-7 milligrams per gram. I've seen studies in which the absorption of minerals like zinc and copper — given a phytic acid level of 4 milligrams per gram in the grain or legume — falls into the general range of 10-30%. When virtually all of the phytic acid is removed from the grain or legume, this range will increase for zinc into the area of 25-40% but will remain essentially unchanged for copper.
While soaking can have some impact on the phytic acid found in oats and can lower this amount somewhat, it cannot remove as much phytic acid from the oats as was accomplished in these scientific research studies. As a result, I do not believe you would be able to increase the availability of minerals from your oats by a very large amount by soaking them overnight. In this context, it is also important to remember that phytic acid is often a plant's key storage form for the mineral phosphorus and for a nutrient called inositol. I consider these nutrient components of phytic acid to be potentially health supportive substances.
For more information on this topic, please see:
Cheryan M.Phytic acid interactions in food systems. Crit Rev Food Sci Nutr. 1980;13(4):297-335.
Garcia-Estepa RM, Guerra-Hernandex E, Garcia-Villanova B. Phytic acid content in milled cereal products and breads. Food Res Int. 1999;32(3):217-21.