Heat 2 tablespoons of the olive oil in a large skillet. Brown the ribs on all sides. You may have to work in batches. Add remaining olive oil to the skillet if needed.
2-3 Tablespoons olive oil
To the slow cooker, add a layer of ribs (start with any that contain bones) on top of the apples and onions. Cover those ribs with about 1/3 cup of the barbecue sauce mixture, brushing on to cover the meat.
Layer remaining ribs on top of the first layer and pour on the rest of the barbecue sauce. Use a brush to ensure all ribs are covered.
Cover the slow cooker and cook on low for 7-8 hours or on high 4-5 hours.
The brand or style of barbecue sauce you use comes down to personal preference. You can make homemade Dr. Pepper barbecue sauce or purchase a bottle of your favorite sauce from the store. I want to say it doesn't matter, but it really does when it comes to ribs. You want to use a sauce you enjoy the flavor of, because that's one of the main flavor components here.
If liquid smoke isn't an option for you, a good substitute is to replace the regular paprika with smoked paprika. But honestly it's not necessary unless you really want that smoked flavor.