In a medium bowl, whisk together the olive oil, red wine vinegar, minced garlic, oregano, salt, pepper, and sugar until well combined and emulsified. Let the dressing sit for a few minutes to allow the flavors to meld together. I find that freshly minced garlic makes a noticeable difference in the dressing's brightness, so don't skip that step if you can help it.
While the dressing develops, prepare all your vegetables: dice the cucumber into bite-sized pieces, chop the tomatoes into chunks, thinly slice the red onion into half-moons, dice the green bell pepper into similar-sized pieces, and roughly chop the parsley. Having all your vegetables prepped and ready makes assembly quick and prevents the avocado from oxidizing while you're still chopping.
In a large serving bowl, combine the cucumber, tomatoes, red onion, green bell pepper, kalamata olives, and parsley from Step 2. Pour the vinaigrette from Step 1 over the vegetables and toss gently to coat everything evenly. Let the salad sit for 2-3 minutes so the vegetables can absorb some of the dressing and the flavors blend together.
Just before serving, gently fold in the cubed avocados and 1 cup of the feta cheese into the salad. Add the avocado at the end to prevent it from breaking down or oxidizing. Top the salad with the remaining 1/4 cup of feta cheese for a beautiful presentation and extra flavor.