Heat large sauté pan over medium heat and add olive oil. Saute onion until soft and translucent, about five minutes.
Add garlic, oregano and cumin and stir for one minute.
Add corn, beans and salt (if using) and cook, stirring occasionally, for 5-10 minutes to allow flavours to blend. Taste and adjust seasoning if necessary.
Remove from heat and place in a large serving bowl. Toss with tomatoes, cheese, parsley and top with avocado.