I don't think the Onion was originally meant to sound real though. It's just gotten that way because of how absurd our actual current events are.
Similar to how the "Weekend Update" segment on SNL used to be primarily joke headlines (with the exception of the OJ Simpson trial), but now they can literally make an entire segment using actual footage from a Trump rally.
The Onion, and parody in general, aims at being an exaggerated caricature of the truth, something that has a kernel of plausibility in its core, but has elements or scale that is blown out to ludicrous extremes, or is framed in an unexpected way such that it inherently juxtaposes a social approach to a certain realm of topics with another realm of topics, exposing a double standard that larger society has.
Parody is not just 'plausible, but fictional.'
That would ... just be a hoax, mis or disinformation, or realistic fiction if not presented as news.
... The fact that it is so difficult now to tell many parodies from a person's or group's actual actions or statements, or complex events... this speaks to an increasingly extreme world, it speaks to ... basically the death of a general concept of 'normal' from which parody can grow.
Unless you really want to split hairs on a per article basis, in general, it is both.
If your subject for parody is 'news about current events', then you can argue that anything presented in the format you would expect from a newspaper, but comically exaggerated, counts as parody.
If a certain Onion article is fairly clearly a modification or exaggeration of a specific news event, it fulfills another common attribute of parody by having a specific thing it is parodying.
But sure, some of their stuff falls more into satire.
Satire more often has a more ... thorough and deliberate alteration of a base material or style, with the intention of conveying a more intentional critique of or commentary on the source, a fairly obvious prescriptive moral message, by subverting it in specific ways...
... as opposed to parody, which, by itself, doesn't really need to have a 'message', it can be just a level of exaggeration that is humorous in and of itself.
Generally, most of the Onion's produced content is both parody and satire.
you know I was prepared to argue the point but I realized, the articles I enjoy and save are the satirical ones. there are a lot that are just plain parody that I just skim over. But they had to call themselves parody in the best legal brief ever written, dammit.