In a 6 quart Dutch oven or soup pot, heat olive oil over medium high heat. Add onions and cook for 4 minutes, stirring constantly. Add garlic and cook until fragrant (about 30 seconds), while continuing to stir.
Add chicken broth, tomatoes with juice, oregano, salt, pepper and bay leaf. Stir well to combine. Bring to a gentle boil and reduce heat as necessary to maintain a simmer (medium low to low).
Cover and simmer for 15 minutes. Remove and discard bay leaf. Carefully purée to desired consistency using an immersion blender. If you don't have an immersion blender place soup in small batches in a blender or food processor (exercise extreme caution while blending hot liquids - blend small amounts at a time and hold down the top cover of blender pitcher or food processor while blending because there is a risk of the top blowing off and hot liquids to blow out!).
Top with desired toppings when serving, such as chopped fresh basil, parsley, croutons and/or Parmesan cheese.