Edit: Found the registration. It’s a weird combined Phase I and II: https://clinicaltrials.gov/ct2/show/NCT04324606
I was rejected because I had cancer (years ago but they don't need any extra factors) but this is mostly like a normal Phase I in that the primary question is: Do humans tolerate this vaccine? Roughly half those recruited (at random) get a different vaccine, an irrelevant one, that protects against Meningitus but we've no reason to think would make any difference to a coronavirus - and half get the new one. You can't tell what it was they stuck in your arm, so they can compare the two groups to see if any side effects were significantly more common for the new vaccine.
Because this coronavirus is widespread in the community they will also unavoidably get to collect information that would usually be the purpose of Phase II/III trials - did the group who got the "real" vaccine get COVID-19 symptoms less often that the other group? Or if they did, were they less severe?
You normally don't do this, and if you tried an ethics committee would stop you - it's unethical to expose patients to a novel risk when you haven't even figured out if the treatment you're trying is safe first. But in this case the risk is already there, for absolutely everyone, so you aren't making anything worse by just measuring if your treatment helped.
In this case, it looks like the Oxford group had previously used the same ChAdOx platform to make a MERS vaccine, and they’re leveraging that data (here: https://www.thelancet.com/journals/laninf/article/PIIS1473-3...) to move more quickly into a combined Phase I and II trial.