I tried the plugin “Recommend Similar Articles” with a journal and it seems that the system uses the AND operator between keywords to retreive similar articles. Only articles with the exact same keywords are showed in the section Similar Articles if the article has more tahn 2 keywords. For exemple, an article with 2 keywords (ex: open access + journals) will show in the section Similar Articles only the articles with the keyword “open access + journals”, not the articles containing only the keyword “open access”.
I would expect the system to use the OR operator to find all articles with similar keywords, not just article with the exact two or three keywords. It even more problematic for the articles who has only one keyword.
I use OJS 3.3.0-8.