Heat the olive oil in a large pot over medium heat. Add the chopped onion and cook until it softens, about 5 minutes.
Stir in the minced garlic, cumin, and oregano. Cook for 1 minute until fragrant.
Add the rinsed lentils, diced tomatoes with their juice, and vegetable broth. Bring the mixture to a boil.
Reduce heat to low, cover, and simmer for 20 minutes.
Add the diced zucchini, salt, and pepper. Simmer uncovered for 10 more minutes until the zucchini is tender and the lentils are cooked through.
Remove from heat. Stir in most of the torn basil and ladle into bowls. Top with the remaining basil.