UC OA Dissertation Policy: Several UC campuses have established policies requiring open access to the electronic theses and dissertations (ETDs) written by their graduate students. As of March 25, 2020, there is now a systemwide Policy on Open Access for Theses and Dissertations, indicating that UC “requires theses or dissertations prepared at the University to be (1) deposited into an open access repository, and (2) freely and openly available to the public, subject to a requested delay of access (“embargo”) obtained by the student.”
In accordance with these policies, campuses must ensure that student ETDs are available open access via eScholarship (UC’s open access repository and publishing platform), at no cost to students. By contrast, ProQuest, the world’s largest commercial publisher of ETDs, charges a $95 fee to make an ETD open access. Institutions worldwide have moved toward open access ETD publication because it dramatically increases the visibility and reach of their graduate research.
From Rachael Samberg's (UC Berkeley) FAQ for Copyright and Publishing Your Dissertation.
Certain publishers in particular disciplines may consider dissertations to be prior publications, and/or limit their consideration of a subsequent journal article or book manuscript based on the dissertation. Some authors may therefore wish to embargo due to concern that open access availability will impact consideration of subsequent publications derived from their dissertations.
Note that this is untrue for the majority of publishers.
Ultimately, you should check with the guidelines of the publishers you are considering. It is also important to familiarize yourself with the policies in your field.