Monster Oatmeal Chocolate Chip Cookie Bars

Name: Monster Oatmeal Chocolate Chip Cookie Bars
Prep time: 10 min
Cook time: 20 min
Total time: 30 min
Ingredients: 2 1/2 cups old fashioned oats; 2 cups all-purpose flour; 1 cup brown sugar (packed); 1/2 cup granulated sugar; 1 teaspoon baking soda; 1 teaspoon salt; 1 cup canola oil; 2 large eggs; 1 tablespoon vanilla extract; 1 1/2 cups semi-sweet chocolate chips; 1/2 cup Reese’s pieces or M&M’s

Chewy, slightly rustic bars studded with semi-sweet chocolate chips and colorful Reese’s pieces, where old fashioned oats give a hearty chew and brown sugar adds a caramel note. The surface browns and the candies add little pops of color and crunch.

This is a one-bowl, no-chill bake that comes together in about 30 minutes from start to finish—mix, press into a 9×13 pan, bake 18-20 minutes, cool, and slice. They’re ideal for Halloween treat trays, bake sales, or an easy make-ahead snack.

Why You’ll Love These Bars

No-fuss, one-bowl mixing and a no-chill dough make these cookie bars fast and portable. The oats keep the texture chewy while the candy and chocolate chips add pockets of sweetness and color—perfect for a Halloween spread or packing for school bake sales.

Ready in about 30 minutes—quick prep and a short bake.
One bowl, no chilling required—minimal cleanup.
Chewy texture from old fashioned oats with plenty of mix-ins for fun bites.
Baked in a 9×13 for easy slicing and portable servings.

If you love no-fuss, mix-in–heavy bars like this, you might also enjoy richer, jumbo-style versions such as Monster Cookie Bars.

Ingredients You’ll Need

2 1/2 cups old fashioned oats: Old fashioned oats add chewy texture and structure so the bars hold together while remaining tender.
2 cups all-purpose flour: All-purpose flour provides the bulk and structure for the bars, balancing the oats and preventing them from spreading too thin.
1 cup brown sugar, packed: Brown sugar (packed) brings moisture and a mild caramel flavor that deepens the bars’ chew.
1/2 cup granulated sugar: Granulated sugar adds sweetness and helps create slightly crisper edges when baked.
1 teaspoon baking soda: Baking soda gives a subtle lift so the bars aren’t overly dense while still staying chewy.
1 teaspoon salt: alt enhances the overall sweetness and balances flavors—use regular table salt for best results.
1 cup canola oil: Canola oil provides fat for tenderness and keeps the bars moist without needing to melt butter.
2 large eggs: Eggs bind the mixture and add structure so the crumb holds together after baking.
1 tablespoon vanilla extract: Vanilla extract rounds the flavor and complements the chocolate and brown sugar notes.
1 1/2 cups semi-sweet chocolate chips: Semi-sweet chocolate chips create pockets of melted chocolate throughout the bars.
1/2 cup Reese’s pieces or M&M’s: Reese’s pieces or M&M’s add color, a little crunch, and a festive finish—split between the batter and the top for even distribution.

How to Make the Bars

Preheat the oven to 350 degrees F. Grease a 9×13 inch baking dish with butter or line it with parchment paper so you can lift the bars out cleanly after baking.
In a large mixing bowl or the bowl of a stand mixer, combine the old fashioned oats, all-purpose flour, brown sugar (packed), granulated sugar, baking soda, salt, canola oil, eggs, and vanilla. Beat until the dough is moist and all ingredients are combined—the dough will appear crumbly but should hold together when pressed.
Stir in the semi-sweet chocolate chips and 1/4 cup of the candy pieces so mix-ins are distributed through the dough. Press the dough evenly into the prepared 9×13 pan, keeping the surface compact and level. Scatter the remaining 1/4 cup candy pieces over the top for a festive finish.
Bake at 350 degrees F for 18-20 minutes, until the edges are set and the bars are golden. Remove from the oven and let cool completely in the pan—cooling ensures clean slices and that the crumb firms up before you cut into roughly Servings: 20 bars.

Monster Oatmeal Chocolate Chip Cookie Bars close-up view

Candy Mix-In Strategy

The recipe uses 1/2 cup candy split so you get both pockets of candy inside and a colorful top. Mix 1/4 cup into the dough and reserve 1/4 cup to scatter on top before baking for the best distribution and presentation.

1/4 cup candy folded into the dough keeps mix-ins in every bite.
1/4 cup on top creates a festive, visible finish after baking.
Swap ideas: use mini chocolate chips, white chocolate chips, or chopped candy—keep the total at 1/2 cup so texture stays balanced.

Press the Dough Firmly

Because the dough is crumbly, firm, even pressure is the key to uniform bars. Press until the dough is compact and level across the pan.

Use the flat bottom of a measuring cup or a sheet of parchment to press without sticking.
Pay attention to edges—pressing them a bit firmer helps prevent over-browning along the rim.
If the dough sticks to your hands, slightly wet your palms or use a piece of parchment between your hand and the dough.

Bake Time & Doneness

Bake at 350 degrees F for 18-20 minutes and watch for visual cues rather than relying solely on the clock: the edges should be set and the top should take on a light golden color.

Edges set and a golden top are the best doneness indicators.
The center may still be slightly soft when you remove the bars—cooling in the pan finishes the set.
If your oven runs hot, check at 16 minutes; if underfilled or using a smaller pan, the bake time may increase slightly.

Tips for Success

Accept the crumbly dough: The dough will be crumbly—that’s normal. Pressing firmly and evenly into the pan compacts it so the bars bake into a cohesive slab.

Pressing tool: Use the bottom of a measuring cup or a piece of parchment to press the dough evenly and avoid finger dents.

Line the pan: Line the 9×13 with parchment and leave a little overhang for easy removal and cleaner slicing after cooling.

Warm bars: For a warm, gooey bite, microwave a single bar for 8–12 seconds—don’t overheat or it will become too soft.

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Frequently Asked Questions

You can, but quick oats produce a finer, less chewy texture. Old fashioned oats are recommended to retain the cookie-bar chew described in the recipe.

The recipe is formulated for a 9×13 pan to yield Servings: 20 bars. Using a smaller pan will make thicker bars and require a longer bake time—watch the edges and top for the doneness cues provided.

Yes. Cool completely, slice, then wrap individual bars tightly or store in an airtight container with parchment between layers. Freeze up to 2 months; thaw at room temperature before serving.

Ready to Bake

These Monster Oatmeal Chocolate Chip Cookie Bars are a fast, crowd-pleasing bake—mix in one bowl, press into a 9×13, and you’ll have portable, chewy bars perfect for Halloween or any party.

Make them ahead for easy transport and slice after cooling for tidy bars you can pack or display.