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In order to change Organization Owner, make sure you are part of Project Collection Administrators Group below:
In Azure DevOps Organization > Organization Settings > Permissions > Project Collection Administrators > Members > Add yourself as a user > Save.
If you don't have access, ask someone who's a member of Project Collection Administrators group to grant you access.
Once you are part of the group > visit > Organization Settings > Overview > Change Owner > Select your user > Click on Change: