I would like for my planet to be close to Earth’s size (so similar gravity), with much of the surface covered by water, containing atmospheric oxygen from abiotic processes (the photocatalytic reaction of titanium oxide on the planet’s surface perhaps?), a comfortable temperature (so most likely there is CO2 in the atmosphere), maybe around 0.5 AU from the star, but I’m open to anything that’s realistic. The ideal atmosphere would be “almost” breathable, some trace poisonous gases are fine, too little oxygen is also okay (my characters have ventilators, and possibly technology which can extract and concentrate oxygen gas from the air). I’m happy to tweak any parameters necessary. Thanks so much!
Despite the ability of titanium dioxide to breakdown water into hydrogen and oxygen, I doubt very much that this reaction would be sufficient to provide more than a trace of oxygen.
The problems are many fold, the reactivity of oxygen, the blocking effects of the atmosphere, the formation of ozone and the weathering or covering of of exposed TiO2 by other rocks or deep water.
So the best option might be to have some form of concentrator to extract low levels of oxygen from the atmosphere.
Note if by some means the environment you describe did exist abiotically, it would not remain abiotic for long once people had arrived. Even if using fully pressurised suits (probably not required) some bacteria and other life forms would escape and multiply rapidly.
Since you mentioned oxygen deprivation might be something you want to explore for this planet, I managed to find an article with detailed information on it, from OHSHA.
I can tell you from high altitude diving, that oxygen deprivation is a serious problem. It can render a crew so senseless, that they start pushing each other out of the plane, wrestling for controls, and all sorts of idiocy that it's a wonder anyone survived. And then, they don't remember a thing after the fact.
For your planet, you could make it so that you have just enough oxygen if you sit and rest... but any exertion which the main characters perform will put them in danger of hypoxia, in an environment where you need to keep your wits about you (especially if there are predators).
This could be quite exciting for a story, the tension of being in danger even when you're safe, and in such a sneaky manner at that. If you want an element of tragedy, you could mention that background characters keep underestimating the low oxygen, and having accidents, to reinforce the insidious poison... which is no poison at all.
To get this effect for the early stages of visiting, if you want it, probably something like 16% to 17% oxygen in the atmosphere. You might even want 14%, if you figure the people are adapting to the environment; but it might make it less insidious, as they could learn fast there's just not enough oxygen.
Note that there are some mountaineers who wouldn't use oxygen on Everest... they're not common, and they're incredible in their own right (not sure how many are still alive today). Everest has something like 6% oxygen near the top, and about 10% at basecamp. Of course, you're not just living up there as your space explorers will be, and other climbers considered it insane.
If you did want your people to eventually adapt, naturally, maybe something like 15% oxygen or a bit more. Technology could change this considerably.
Source and Quotes on Oxygen
when the oxygen level of their breathing air drops below [19.5 percent oxygen]. Below 19.5 percent oxygen . . . , air is considered oxygen-deficient. At concentrations of 16 to 19.5 percent, workers engaged in any form of exertion can rapidly become symptomatic as their tissues fail to obtain the oxygen necessary to function properly. Increased breathing rates, accelerated heartbeat, and impaired thinking or coordination occur more quickly in an oxygen-deficient environment. Even a momentary loss of coordination may be devastating to a worker if it occurs while the worker is performing a potentially dangerous activity, such as climbing a ladder.
Concentrations of 12 to 16 percent oxygen cause tachypnea (increased breathing rates), tachycardia (accelerated heartbeat), and impaired attention, thinking, and coordination (e.g., Ex. 25-4), even in people who are resting.
At oxygen levels of 10 to 14 percent, faulty judgment, intermittent respiration, and exhaustion can be expected even with minimal exertion (Exs. 25-4 and 150). Breathing air containing 6 to 10 percent oxygen results in nausea, vomiting, lethargic movements, and perhaps unconsciousness. Breathing air containing less than 6 percent oxygen produces convulsions, then apnea (cessation of breathing), followed by cardiac standstill. These symptoms occur immediately. Even if a worker survives the hypoxic insult, organs may show evidence of hypoxic damage, which may be irreversible
I hope this was helpful. Good luck with the story!