Place the olive oil, vinegar, oregano, salt, and pepper in a small jar with a tight-fitting lid. (Alternatively you can use a small bowl and a whisk.)
Shake or whisk to combine.
Taste and adjust the seasonings, as needed.
Refrigerate until ready to use. Shake well before using.
Notes
Dressing will keep refrigerated in a jar or bottle with a tight-fitting lid for up to 2 weeks. Shake well before using. If the olive oil has solidified, let the dressing sit at room temperature for a few minutes until it liquifies.