Once the juice is squeezed and stored in gigantic vats, they start removing oxygen. Why? Because removing oxygen from the juice allows the liquid to keep for up to a year without spoiling. But! Removing that oxygen also removes the natural flavors of oranges. Yeah, it’s all backwards. So in order to have OJ actually taste like oranges, drink companies hire flavor and fragrance companies, the same ones that make perfumes for Dior, to create these “flavor packs” to make juice taste like, well, juice again.

<https://gizmodo.com/dirty-little-secret-orange-juice-is-arti...>

And that's for product actually marketed as orange juice, rather than, say, a sweetened citrus-flavoured beverage, for which the first ingredient often is sugar and/or corn syrup, e.g., "True Orange Mango Orangeade":

Ingredients: Cane sugar, crystallized orange (ctiric acid, orange juice, orange oil), natural flavor, stevia leaf extract, beet juice and beta-carotene.

(Beet juice and stevia are both sweeteners, the first is effectively liquid sugar, the second is a non-caloric / low-caloric non-sugar sweetener.)

<https://www.truecitrus.com/products/true-orange-mango-orange>

The fact that you're better off either squeezing your own or better yet eating whole fruit is a whole 'nother matter.

A more detailed production process:

"How products are made: Orange juice":

<https://www.madehow.com/Volume-4/Orange-Juice.html>