There isn't enough detail in your question to definitively find the article in question. However, we can give general advice to address "Looking for article..."
PERSI, the PERiodical Source Index, is maintained by Allen County (Indiana) Public Library. PERSI "provides more than 3 million citations to readily-available periodical sources." These periodicals include international, national, and regional (state or county within the US) genealogy publications. The time frame covers current issues of extant periodicals as well as back issues of defunct ones.
You can use PERSI to search for articles by surname, geography, or keyword. Note that PERSI does not index the full text of an article, but merely the subject terms that are associated with the article. Therefore, if the surname of interest is merely mentioned in an article and isn't the main focus of the article, it probably won't be indexed for that article, so you may find that a geography search yields better results.
For your example, since Oliver Cromwell is a well-known historical figure and you're looking for an article you have read before, this would be a good case where a surname search would be useful. You could perform a surname search for "Cromwell," optionally filter the results, and then skim the remaining article titles to see if anything rings a bell.
Once you have found an article that looks interesting, you probably want to locate the periodical. I usually search in these locations in this order:
- My local genealogy library's catalog
- My local genealogy library's periodicals finding aid (because many of the bound periodicals are not in the catalog)
- FamilySearch catalog
- FamilySearch books
- WorldCat
If your library has a physical copy of the periodical that you can access, it helps to know how periodicals are shelved at the library. Here's one way to do it; your library may arrange its holdings differently:
Current periodicals usually arrive at the library a single issue at a time, as they are published. These might be displayed in a vertical rack with other unrelated current periodicals and not necessarily near the other materials about that country/state/county/surname.
Once the library has accumulated a certain number of issues (a year, two years, ...), the library has the issues bound into a book, which may be shelved in the stacks with other books about the country/state/county/surname.
The bound periodicals may or may not be cataloged in the main library catalog like regular books. If they are not, your library may have a separate resource that lists the periodicals. Ask a librarian at your local genealogy library if they have a "periodicals finding aid."
If you have access to a genealogy library whose catalog or finding aid says it has an issue of a periodical you want to access but you can't find the issue, take note of all the information in PERSI and in the catalog or finding aid and take that information to the library reference desk for assistance. That's what they're there for. At a minimum, you'll want to jot down the following information:
- The title of the journal, e.g. "Genealogical Journal" or "Ansearchin' News"
- The publisher or country/state/county of publication, e.g. "Ohio Historical Society" or "Randolph County, North Carolina." Genealogy societies are not always creative with their journal names, and there is a lot of duplication, so having the publisher or location will help narrow down the journal.
- The volume and issue number, e.g. "volume IX, number 3" or "16:2"
- The date of the issue, e.g. "Winter 1987" or "July 13, 2002"
- The article title and/or page number(s), if provided
For more information about PERSI, the FamilySearch Wiki page on PERSI has instructions and tips for using PERSI effectively and for obtaining articles you've located. Cyndi's List also links to some how-to articles about PERSI.