Overall, we had a positive experience with the van and the hosts. However, there are a few things that people should know before thinking renting a '#vanlife' van on O'ahu is an easy, inexpensive alternative to renting a car and getting stationary accommodations.
First: it is technically illegal to sleep in your vehicle in streets or public property (according to §291C-112, available at capital[dot]hawaii[dot]gov). The state parks on the island are all specifically designated for tent camping, so that's not a legal option, either. None of this was pointed out on the listing, nor highlighted by the folks renting the van out, until we raised the issue. In practice, though - it may be that police won't knock on your window when sleeping at a few places if you keep to yourself and don't bother anyone. One of these places is Sandy Beach on the east side. You can also park a tent out by your van in the state parks (up to you if you decide that fulfills the letter of the law).
Second: after extensive research we had to conduct, there are (as of this writing) only two private, overnight campsite that specifically allows vehicle camping - Malaekahana beach campground, and Friends of Malaekahana (which appears to be for religious group outings, and expensive). Both places fill up quick.
Third: communication with the host before arrival was very piecemeal and delayed. It seems like the host is very busy with other things, and was actually off-island at one point while still renting out the van, based on what he mentioned to us and based on other reviews. Very nice and chill guy and ultimately we had no issues as a result, but concerning when unresponsive for weeks at a time before arrival.
Fourth: some stuff is the van was broken, and we weren't informed until arrival. The gauge for how much battery power is left was inaccurate, there appeared to be a loose or corroded connection in the solar charger panels, the power window on the driver's side door didn't work, and the camping chairs provided in the van were on the verge of tearing. Additionally, the Airbnb listing was confusing, as it reports toilet availability, but that is an extra cost added item (and was basically a bucket), with a separate listing on a separate RV rental website for this same van with information that was contradictory to this one (availability of lawn games, knick-knacks, and other small things). Pricing for things like surfboards (mentioned in the listing) wasn't mentioned until the day of rental, although the fees were reasonable.
Finally, there's stuff non-locals should really think about before renting a van to life out of on this small island. None of it is the host's fault, but beware: parking the thing anywhere significantly populated, or - god forbid, Honolulu - is nigh impossible. It eats gas because of the slow speeds and stop-go traffic through the highways around the island. The van has to stay turned on to leave the AC on (there's no separate unit running of electricity), which is pretty important overnight when parked during the summer somewhere that isn't on a breezy beach. The rooftop fan isn't particularly effective without the door wide open, as it blows air in from the top, rather than exhausting it.
Overall, our stay in O'ahu was great - there's just some stuff we wish we'd known before making the decision to rent a van, and we wished the host was more quickly responsive before our arrival.