If your journal has been configured for DOIs, you can easily determine what the DOI will be before you publish it. This way, you can add the DOI to the even though it is not possible to view the DOI before publication.
See the chapter on the structure of DOIs to determine what a DOI will be and to transform the DOI into a URL. Once you have determined the DOI and transformed it into a URL, you can add the DOI to the galley as a link that will go live once the submission is published.
The galley is the final form of the article - editing, copy edited, and laid out - that is posted to the OJS site for reading.
Along with issues, submissions are a key element in OJS. "Submission" is a generic term that includes articles, book reviews, and art. In the back end, a submission has a Workflow and a Production tab. Once published, a submission has its own web page with metadata and one or more galleys.