Oobsidian is a rare crafting material in Ooblets used to build and upgrade Oobcoops, homes for excess Ooblets to live in when they are not with you. In order to progess in Ooblets you will need to find a lot of Oobsidian so you can build and upgrade Oobcoops to expand your Ooblets storage and make a self-sustaining farm. Oobcoops can be crafted by visiting the Manatwee shop's frabricuter and giving him 3 Clothlet, 15 Nurny, 10 Planklets, and 1 Oobsidian.
You will be able to find Oobsidian multiple ways, these are:
How to get Oobsidian
Sea Dangling Sea Dangling is basically fishing and can reward you with Oobsidian or other good items.
Breaking Rocks Breaking rocks on your farm each day can get you Oobsidian. Make sure you leave enough open space on your farm for rocks and other items to spawn each day.
Assign Sparkly Ooblets to Oobcoops Putting Sparkly Ooblets in Oobcoops can sometimes give you a piece of Oobsidian while it is busy working around the farm.
Search People's Homes You can find Oobsidian, as well as other useful stuff such as Recipe Fragments, Gummies, Club Tokens, and even food by searching through peoples homes in Badgetown. Oobletes do not care if you go rifling through their junk.
Buy them with Wishies It is advised that you do not do this due to how expensive Oobsidian is but you can also buy it with Wishies. As you progress through Ooblets you will earn Wishies, a special currency that can be turned in via the Wishy Well in the center of Badgetown.