As you continue to progress through Ooblets, this resource will often be required. Already, it is needed to build Oobcoops which are important to help your farm progress. Oobsidian is much harder to find compared to other materials. Luckily, there are multiple ways in which you can earn this pretty rare resource.
Collect Oobsidian from gleamy Ooblets
If you have managed to collect a gleamy Ooblet while exploring Badgetown, they can be useful in earning you some Oobsidian. Assigning a gleamy Ooblet to work on your Oobcrops will not only progress your farm, but these Ooblets have a chance to drop Oobsidian while they work. This means you should frequently check on your farm and see what resources your Ooblets leave behind.
Try smashing rocks
Every day, rocks along with logs and weeds will begin to overrun your farm. Rocks in particular have a chance to give you some Oobsidian. Simply just smash the rocks and see if you earn any reward. Although they often give Nurnys, there is a chance that you can earn some Oobsidian. Also, rocks are able to spawn around Badgetown, so you should definitely look out for those too.
Fish for the rare resource and go sea dangling
Another potential way to get your hands on some Oobsidian is to go sea dangling. To do this, you need some bait. You can create some bait by recycling rubbish at the Reconstitooter machine. Following this, you can head to the pier and take part in some sea dangling and hope you catch Oobsidian.
Use Wishies at the wishing well
If all else fails, you can always use your Wishies to buy the Oobsidian you need. However, they are quite a pricey investment, with a single piece of Oobsidian costing 100 Wishies. Overall, it is a useful resource in Ooblets that is needed towards more complex projects throughout the game.