: Should items separated by commas be alphabetical? When writing a list of items, should they be alphabetical? The late Jurassic periods are Tithonian, Kimmeridgian, and Oxfordian. better? The late

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No. And particularly not in your example of a list of historical periods, in which the obvious listing would be in chronological order. Alphabetical order would be perverse! In most other cases, it would be merely unnecessary.
Also, you misunderstand what the 'Oxford comma' is. In 'The late Jurassic periods are Kimmeridgian, Oxfordian, and Tithonian.' the Oxford comma comes before 'and'. The rest are just commas.

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There's nothing right or wrong with alphabetical order for nouns. Like a lot of English, there's no explicit rule but there's a way that sounds best. It often depends on what the items are. If history is involved, chronological order makes sense. If they're items you'd find on the shelf at an auto parts store, sorting by function helps the reader find them.
If the list is made of adjectives, on the other hand: a dragon that is "great, old, and green" sounds fine, but "old, green, and great" sounds like the speaker is clumsily making it up as they go along. What's happening here is covered in "The Elements of Eloquence" by Mark Forsyth (2013), where he says

"... adjectives in English absolutely have to be in this order: opinion-size-age-shape-colour-origin-material-purpose Noun. So you can have a lovely little old rectangular green French silver whittling knife. But if you mess with that order in the slightest you'll sound like a maniac. It's an odd thing that every English speaker uses that list, but almost none of us could write it out."

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There's no rule about the order of listed elements, so this is not a question of grammar, but of style.
There are a few different approaches you could take:

An alphabetical order might make it easier for readers to remember the elements. (However, if it's about ease of memorisation, a better approach might be to see if one particular order creates a memorable word when putting the initials together, and use that.)

Sometimes, you can go by what sounds best. I know this is really subjective, but for me it usually involves the starting letter of each word and the number of syllables. In your example, "Oxfordian" and "Tithonian" have the same number of syllables, so if it's a matter of style, I wouldn't squeeze the longer one between them.

Often, lists get sorted in order of importance, by either listing the most or least important first. For example, if the rest of the paragraph were to focus on one of these periods, I would list that one last.

However, in your example, you are talking about geological eras, so I would actually sort them chronologically. Maybe you could describe this as the order that "makes sense". (For example, in this related question, the asker suggests ordering articles of clothing from head to feet.)

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