Mash avocado in a bowl using a fork or potato masher. Stir garlic salt, lemon juice, and oregano into mashed avocado.
Arrange split croissants on a work surface. Place 2 slices turkey on bottom half of each croissant. Layer 1 slice Swiss cheese and 2 bacon slices atop each turkey layer. Place each croissant top over bacon layer. Arrange sandwiches on a baking sheet.
Bake in the preheated oven until cheese is melted, 5 to 7 minutes.
Open each sandwich and place 2 tomato slices and 1 lettuce leaf atop bacon layer. Lightly spread 1 teaspoon mustard and 1/4 the avocado mixture onto each croissant top. Return each top to each sandwich.