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Preheat the oven to 325°F. Lightly grease an 8" square pan and line with a parchment paper sling.
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To make the cookie base: In a medium bowl, whisk together the flour, salt, and baking soda until well-combined.
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In a separate large bowl or the bowl of a stand mixer, beat together the butter, sugars, spices, and vanilla, mixing until smooth.
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Beat in the eggs one at a time, scraping the bowl between additions, until the mixture is smooth, creamy, and thick.
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Stir in the flour mixture until just combined, then mix in the oats.
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Transfer the dough to the prepared pan and smooth it into an even layer with a flexible spatula or an offset spatula.
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Bake the oatmeal cream pie bars for 35 to 40 minutes, until the color is a deep golden brown and a toothpick or paring knife inserted into the center comes out clean. Cool for 10 minutes in the pan on the wire rack, then lift out of pan and let cool completely on the wire rack.
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To make the frosting: In a large bowl or the bowl of a stand mixer, beat together the shortening or butter, sugar, and marshmallow until well combined. Add the vanilla, water, and salt and beat until smooth.
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Spread the frosting on top of the cooled bars, top with sprinkles, and slice into 16 pieces.
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Storage information: Store oatmeal cream pie bars in an airtight container at room temperature for up to 3 days. To make ahead, freeze baked (unfrosted) bars, then thaw in the refrigerator overnight. Top with frosting when ready to serve.
