Heat the oven to 350 degrees. Line a 8-inch square baking pan with parchment paper or grease with oil spray.
In a large bowl, whisk together the pumpkin, sugar, oil, honey, and vanilla. (If you measure the oil before the honey, the honey will easily slide out of the measuring cup.)
Whisk in the pumpkin spice, salt, chia seeds, and ground flax.
Stir in the oats using a large spoon or rubber spatula. Mix until all of the ingredients are well combined.
Spread the oat mixture evenly in your prepared baking pan. I like to use the back of a spoon to get into the corners and smooth the surface along the edges.
Bake until the edges are very golden brown, 30 to 35 minutes. Allow to cool fully before cutting into bars. I like to cut into 5 equal pieces in one direction then in half to make 10 bars.
Bars are best stored in an airtight container in the refrigerator for 5 days or individually wrapped and frozen for up to 2 months.