Set one of the oven racks on the lowest shelf and heat oven to 350ºF.
Line a half-sheet pan with parchment paper and set aside. Make 8 (2-inch) balls out of aluminum foil. Coat 3 wire racks with nonstick spray and place one of the aluminum balls in each of the corners of 2 of the racks.
Lay one-third of the chips on one of the racks. Top each chip with a jalapeno slice and sprinkle the chips with one-third of the red onion and one-third of both of the cheeses. Repeat with the remaining chips and the other 2 racks.
Stack the racks on top of one another and set in the parchment-lined half-sheet pan. Bake in the oven on the bottom rack until the cheese begins to bubble, about 7 minutes. Sprinkle with the oregano. Serve immediately with salsa, sour cream, and guacamole.