Given that there is plenty of new healthy growth and the yellowing is only on older leaves I'd guess it is caused by root disturbance from being transplanted in full growth. Only you can tell if it is over or under watered by digging down and seeing if the soil is moist at the roots. I notice the foliage is wet. If that is due to overhead watering it would be best to stop that and water only around the base.

I also live in Southern California. I bought these two fruitless olives at the same time 4-5mo ago, but the one on the right is turning yellow. Both are still in their original pots and drain into the decorative pots that also drain and are not waterlogged. I don't water them differently but the green one on the left gets slightly more shade. Should I repot? Feed? Please tell me what I need to do. Thanks in advance.

Thanks for the reply ! We had some rain yesterday hence the leaves being wet. How often should we be watering?

Here's a good article on growing and care of fruitless olive trees.

Once established, they don't require frequent watering.

https://www.thespruce.com/fruitless-olive-tree-profile:

" How often should we be watering? " No plant of any kind should be watered on a schedule. You need to provide water when the plant needs it and only you can determine that, through observation and experience. Given it's a drought resistant type getting a bit thirsty occasionally won't kill it. Better than waterlogging.