Obsidian doesn't use Open Source Licensing. It's effectively a non-commercial license, labeled as "free for personal use":
Obsidian is free for personal and non-profit use. However, if you use Obsidian for work-related activities that generate revenue in a company with two or more people, you must purchase a commercial license for each user. Non-profit organizations are exempt from this requirement. https://obsidian.md/license
Later on, it clarifies that single users can use it under a no-charge license, even for work:
De Minimis Commercial Use License, for sole-proprietorships and single-person organizations
And finally, the Catalyst license is a one-time charge to support development, with early access and insider builds.