If it is freshly squeezed or classified as pure unfortified 100% orange juice it is vegan.
Unfortunately, albeit with what I can only assume are the best of intentions, many fruit juices are fortified with Vitamin D, which can be sourced from plants but is more often sourced from fish oil or lanolin (from sheep’s wool).
It is often also fortified with omega-3 fatty acids which can be sourced from algae oil but again are more often sourced from fish oil or even sometimes gelatin.
Is orange juice healthy? And do I need it in order to get enough vitamin C?
Orange juice is rich in important nutrients like potassium, folate, 6% of your daily recommended amount of calcium and, of course, vitamin C, but you can get those nutrients in plenty of other foods, and by eating an actual orange!
In fact, despite being nutritionally similar to whole oranges, orange juice provides very little fibre but twice the calories and sugar. Like all fruit juices, it can cause blood sugar spikes and even weight gain and is especially not the best choice if you are trying to manage your blood sugar levels or have diabetes.
I’m not saying you should never have a glass of orange juice, I’m just saying that aside from the occasional small glass, you’re better off just eating the orange. BTW, there are plenty of other sources of vitamin C you know… for example: