The young require lots of live foods such as amphipods, isopods and mysid shrimp. Some species like O.bimaculoides can be batch reared. O.briareus hatchlings need to be reared in seperate rearing chambers or the will slowly eat each other. Pygmy octopus like o.mercatoris are a little more tricky due to their small size but they will actually do ok for a week or so on enriched brine shrimp untill you can order or catch the required amount of live food. Artemia will not work for the other species however. Nutrition is critical for the first few months and the food needs to contain lots of HUFAs and they prey needs to be enriched with various mico algaes like nanno, isocrysis.... It can be done but it takes total devotion and lots of live food and high water quality.
I live close to the ocean so it was easy for me to get live food when I reared mine. I also had quite a few buckers and tanks of mysids and amphipods being cultured. It's alot of fun but very time consuming and can get quite costly.