Heat the oil in a skillet over medium-high heat and brown the beef until no pink remains, 7 to 8 minutes; drain excess fat.
Add the onion and cook until soft, 4 minutes, then add the garlic for 30 seconds.
Transfer the mixture to a 6-quart slow cooker and stir in the tomatoes, ketchup, mustard, Worcestershire, salt and pepper.
Cover and cook on low until the sauce is thick and the onion has disappeared into it, 4 to 4 1/2 hours.
Boil the macaroni until just tender, drain it and fold it into the beef sauce.
Stir in the cheddar by handfuls, cover and cook on high until melted, about 10 minutes.
Fold in the pickles just before serving so they keep some crunch.