It depends on how sour you like your dressing. I tend to go for 3:1 of extra virgin olive oil: high quality vinegar (e.g. cider vinegar, wine vinegar of either colour, balsamic vinegar). If you want to make full-on vinaigrette, take that and add a dab of mustard - which allows the oil (base) and vinegar (acid) to emulsify, some freshly crushed garlic, and some oregano. Or you can replace vinegar with lemon juice - I tend to use either/or rather than both/and. If in doubt, find some basic vinaigrette-based recipes to try and stick with one or two that you like.