Preheat oven to 350 degrees, with racks in upper and lower thirds. In a medium bowl, stir together oats, flour, salt, and baking soda. In a large bowl, using an electric mixer, beat butter and sugars on high until pale and fluffy, 3 minutes. Beat in maple syrup, egg, vanilla, and 1/4 cup water, scraping down bowl as needed. With mixer on low, beat in oat mixture in 2 additions until just combined.
Drop dough in rounded teaspoons, 2 inches apart, onto parchment-lined baking sheets. Bake until edges are golden, about 11 minutes, rotating sheets halfway through. Let cookies cool on sheets until set, about 2 minutes, then transfer to wire racks and let cool completely.