Not always!
There are a few options for publishing OA that don’t require you to pay.
Green OA may allow authors to share their work through self-archiving.
You can attach an Author’s Addendum (such as this one from SPARC) and/or Bucknell’s Open Access Policy to your contract with the publisher. The addendum is typically accepted without negotiation, and you are free to upload your work to Bucknell Digital Commons, your personal site, and/or another digital repository.
Diamond OA journals do not charge Article Processing Charges (APCs).
In the Directory of Open Access Journals, you can filter your journal search to see journals without APCs. Once you identify a journal in the DOAJ, you can use the site, as well as the journal site, to learn more about the review, publication, and editorial processes of the journal.
Keep in mind that Diamond OA journals have not benefited from the visibility or history of many journals that are currently owned by commercial publishers, a limitation that is currently being addressed in the 2022 Action Plan for Diamond Open Access.