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Preheat the oven to 325°F with racks in the upper and lower third. Line two baking sheets with parchment or silicone mats or lightly grease.
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In a medium bowl, combine the oats, flour, baking soda, salt, and cinnamon.
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In the bowl of a stand mixer fitted with the flat beater attachment, beat together the butter and sugars until the mixture is the texture of wet sand, about 1 minute on medium speed.
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With the mixer on low speed, add the eggs one at a time, beating to incorporate each one and scraping the sides of the bowl after each addition.
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Mix in the corn syrup, vanilla, and peanut butter until well-combined and scrape the sides of the bowl.
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Add the oat/flour mixture all at once and beat on low speed just until a couple of streaks of flour remain, about 1 minute on low speed.
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Add the chocolate candies, pretzels, and chocolate chips all at once. Mix until they’re evenly incorporated into the dough, about 30 seconds on low speed. Scrape down the sides of the bowl and the beater, as needed. Finish mixing by hand to ensure no dry pockets remain.
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Use a muffin scoop (1/4 cup or about 63g) to portion the monster cookie dough onto the prepared baking sheets, leaving at least 2 1/2" of space between cookies (8 cookies per sheet). With damp hands, press each dough ball to 3/4" in height.
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Bake for 16 to 18 minutes, rotating front to back and top to bottom halfway through, until edges are set and golden brown but centers are still soft.
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Remove the monster cookies from the oven and let them cool on the baking sheets for 10 minutes, during which they will continue to set up. Transfer the cookies to a rack to cool completely.
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Portion and bake the remaining cookie dough.
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Store leftover monster cookies in an airtight container at room temperature with a slice of plain sandwich bread for up to 5 days. (This helps keep the cookies soft.) To make ahead, freeze scooped and pressed dough balls for up to 2 weeks. Bake directly from frozen (no thawing) as directed in the recipe. An additional minute or two of baking time may be required.
