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It's hard to determine which recipe for "overnight oats" caught your interest. However, the one for steel-cut oats is for a coarsely cut grain sometimes called called Irish oatmeal; it's the opposite of instant oatmeal. Other recipes are for "regular" rolled oats that are processed only by being milled into flakes. The purpose of "overnight-ing" oatmeal is to cut down the time spent preparing breakfast in the morning, something that manufacturers of instant oatmeal believe they have already done for you. I would never substitute instant oats for steel-cut oats. Though you might have to adjust amount of water (less), and cooking time (ditto), you might get away with the substitution. However, results could be a bit gluey-gooey and disappoint. In general, recipes will specify "instant oatmeal" if authors intend you to use instant flakes, usually for cookies.
Thank you, Eliz.
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