Only for personal use. If you're using it at work, you're supposed to purchase a license. https://obsidian.md/eula

> You need to pay for Obsidian if and only if you use it to contribute, directly or indirectly, to revenue-generating, work-related activities in a company that has two or more people.

That's not exactly how I'd read that EULA. To me it's asking for payment if you use it for work not at work. so someone recording personal notes on a work system wouldn't fall into that category, but someone using it for product development at their company would...

Hm. Yeah, I guess it's not as clear-cut. The Commercial License docs are maybe a bit more explicit? https://help.obsidian.md/Licenses+%26+Payment/Commercial+lic...

> If your notes contain content directly related to work projects or processes for a greater-than-one-person company, then you require a commercial license.

(This is from Q13 on that page, "I use Obsidian to store all of my knowledge, both personal and professional, because it's difficult to separate them. Do I need a commercial license?")

ah that's an interesting clarification! They should really give that more prominence as I'd guess there'll be quite a few people with that use-case.

If what you're writing is related to your professional activity, you're supposed to pay. So for example if you use it (as the article suggests) to record your work achievements at your job, you're supposed to buy a license.