For the early middle ages, it can be very difficult to pin down specific information like this. More frequently, researchers investigating a particular site will think in terms of comparisons and patterns. At least in the past, Spanish archaeologists (and they weren't alone) often blew through medieval layers to get to the Roman ruins beneath, which is part of why the "Dark Ages" seem so dark. At present, it seems that Oiasso has had some exciting archaeological excavations of Roman ruins, and these are well presented by a local museum.

It sounds like you've already got some ideas about the patterns and contexts around 10th-century Oiasso, since you've asked some intelligent questions. My off-the-cuff but educated (I hope!) guesses would be: Roman ruins were still standing but had been despoiled (taking stones for building other things) or repurposed. Populations were typically small outside of major centers, so 30-200 might be a decent range of guesses. Norse raiders certainly trafficked (rarely) along the coast and they may have stopped in Oiasso either as raiders or in search of a friendly harbor or temporary (seasonal) base, but they probably wouldn't have settled any longer than a single winter. The Visigoths don't appear to have built many enduring things, so any Visigothic cultural remnants probably consisted of traces of their activities in the old Roman structures (graffiti and reconstruction) and perhaps some crumbling wooden buildings as well. Early medieval kingdoms could be more predatory than territorial—think more like a mob demanding 'protection' money than a state demanding taxes—but it seems like there were complicated politics at a very local level with occasional interventions from Castille, Navarre, the Franks, and various Muslim rulers. You might also consider that this was the years when the Camino de Santiago was first beginning to be traveled, so there might have been a few pilgrims passing through the region as well, although not nearly as many as during the later Middle Ages. I suspect that your gut intuition is correct and this would have appeared more like a fishing (and maybe mining) town rather than a 'city' in any modern sense of the word. By the 900s, the Latin word civitatis more often meant 'place with a cathedral' and didn't necessarily indicate a particularly urban community.

If you'd like to know what was going on in the area, my first go-to source would be Chris Wickham's Framing the Early Middle Ages, which you could treat almost as encyclopedia to look up various ways in which Iberian towns changed during this period. I'm also a big fan of María Rosa Menocal's Ornament of the World, which focuses more on southern Iberia but gives a sense of how vibrant life was across the Peninsula. I also think it's a good complement to David Nirenberg's Communities of Violence, which emphasizes religious conflict in Iberia throughout the Middle Ages.

For further information, the Oiasso Museo has a number of publications, focused mostly on the Roman period but also on museums and public engagement. It looks like they've got an active and thoughtful staff who might be able to answer many of your questions, or at least make educated guesses. I'd recommend reaching out to see if they've got any information about the 900s. It's best to ask general questions (i.e. is there any information on the 900s, such as how the Roman ruins were being used) rather than specifics, at least until you establish a good rapport with researchers who can answer specific questions. I typically write these initial emails in both English and the local language, if I can, so they recognize my ability (if any) in their language and my preference for English. You might also ask for book or article recommendations once you identify who the staff members are who work with medieval materials. I suspect you'd need Spanish to investigate these kinds of questions in much depth.