Brown sausage and onion in large skillet over medium-high 8-10 minutes, crumbling finely. Add garlic in final minute; drain excess grease.
Transfer cooked mixture to slow cooker.
Mix in cream cheese, drained tomatoes, cheese blend, mayonnaise, sour cream, oregano, and optional pepper flakes thoroughly.
Cover; cook on low 2-3 hours, stirring occasionally, until cheese melted and dip hot and bubbling.
Stir thoroughly; garnish with basil or parsley.
Keep on warm; serve with toasted baguette, crackers, or tortilla chips.