Careful of all olive oil in mayo. It can have a strong taste. Olive oil flavor varies, so it depends on what you use. I normally use a blend of olive and canola, but I also use some white wine vinegar and dijon mustard. It was a recipe from a Belgian cookbook. Seems like it was pretty simple ratio-wise: 1 egg yolk, 1 Tbsp dijon, 1 tsp white wine vinegar, salt, white pepper, 1 cup oil. I make it with a whisk, but I also have used a blender. Whisk all the stuff except the oil until thoroughly mixed, add the oil drop by drop to get the emulsion started, and then a slow stream after a bit. Taste and adjust seasoning at end -- very important, use your palate. Egg yolks can vary, so they can handle variable amounts of oil. You can also use lemon juice instead of white wine vinegar. I've made this recipe dozens of times, sometimes adding some Penzey's sweet curry powder, especially when having them with Belgian frites.
I'll use Hellman's if I don't make my own. Mostly I don't make my own if I don't know it's going to get all used. When your minimum quantity is set by one egg yolk, you do wind up with over a cup of mayo from each batch. Kind of a lot if all you are making is a sandwich.