Zest and juice the lemon; grate the garlic - Zest the lemon using a microplane. After zesting, cut the lemon in half and squeeze out the juice. You won't use all of the juice, but I'm sure you can find another use for the excess. Grate the garlic using the microplane.
Make the aioli - Whisk the mayo, oil, oregano, garlic, lemon juice, and zest in a small bowl. Continue whisking until the mixture is silky and smooth.
Notes
You could make your own mayo from scratch, but I find that the extra work just isn't worth the trouble.