Could be bugs...? If you're in the U.S., you could look up images of the damage that four-lined plant bugs cause and compare. I'd say go out at night with a torch and have a hunt under the leaves.

Hawkeye, your oregano looks terribly, desperately nutrient deficient. You need to address that before you do anything else, in my opinion.

Hi rhizo,

One person told me it was just new growth. (There are greener, darker colored leaves elsewhere in the vertical tower.) I personally think it is an iron deficiency. I just added chelated iron last week and was told it may take some time to fully be absorbed by the plant.