I wanna buy olive leaf extract capsules for immune system health and viruses. But I don't know which bottle to order online. Do they all work the same?

Never heard of. I am unaware that olive leaf extract bolster one's immune system. You may be better off by getting your vitamin d3 to the optimal level (60 ng). This can only be assessed by getting a blood test however.
Olive oil. How about just using olive oil in your food and saving the money you are spending on online capsules?