Remove the stems from the mushrooms and wipe them clean with a damp cloth or paper towel.
Rub the mushrooms with olive oil and cook in a skillet or flat grill for approximately 2-3 minutes on each side. You want them to where they are getting soft but are still slightly firm in the middle. Remove the mushrooms from heat and let them sit for a minute, until cool enough to touch.
Pour a 1/2 cup of barbecue sauce into a container or dish of choice; you want one big enough for all six mushrooms to marinate in. Place the mushrooms on top of the sauce, layering if needed. Use hands or a brush to make sure both sides of each mushroom are covered in the sauce. Next, pour another 1/2 cup of barbecue sauce on top of the mushrooms. Let sit for 1 hour, flipping mushrooms once halfway through.
When the mushrooms are almost done marinating, preheat oven to 400°F. Line a baking sheet with aluminum foil and coat with cooking spray. Place mushrooms on the baking sheet, reserving the sauce left in the container or dish. Bake in the oven for 10 minutes.
Remove the mushrooms from the oven and brush them with some of the reserved sauce. Flip each mushroom and then return the baking sheet to the oven for another 5 - 10 minutes.
When the mushrooms are done baking, brush once more with sauce and serve. Enjoy!