Hi Jane, Welcome to Permies!
I have a few young olive trees as well, so I was curious to know what is wrong with your tree. I think it could be Olive Knot Disease aka Olive Tree Tuberculosis, a bacterial infection.
From:
https://en.excelentesprecios.com/olive-knot-disease
During the initial phase small bulges of greenish and smooth surface are formed.
As the development of the bacteria progresses, the bulges grow to a size normally of 2-3 cm. During this phase the warts crush and change color taking brown tones.
In the final phase the tumors crack and darken.
It seems to be spread when rain gets into any wounds on the tree. It's recommended to not prune or harvest when it's wet or there is rain in the forecast. It can spread further when rain washes the bacterium from tumors on the tree to other wounds on the tree. If you are seeing it on the trunk, it may have gotten in through small wounds caused by frost damage. Infections that occurred in winter don't usually show symptoms until spring.
I see a couple freshly pruned branches and a cut on a tumor above one of the branches. If the pruning tool was used on any other trees without being disinfected, it is likely the other trees are infected as well. You could maybe re-prune the other trees, taking off as much of the first cut as possible, disinfecting between each cut. No idea if it would work, but might be worth a try; kind of like taking off the arm with the zombie bite...
Some copper based treatments exist, but are limited to use by commercial growers. Removing infected branches is recommended. If it's on the trunk and it's only one tree so far, it might be best to cut the whole tree down. Preferably on a dry day.
Univ. of California's Olive knot management page with more info:
http://ipm.ucanr.edu/PMG/PESTNOTES/pn74156.html
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