If I spawn without a geyser within 3-4 screens, I stay on algae even with 10+ dupes well within to cycle 70-100+. Algae is everywhere, I've had games where I've had over 50t before I've found a geyser. Placement, I think depends on where the dupes spend most of their time and where the leaks are (if any) and proper gas flow.

I place 1 oxygen source, be it a vent that has oxygen supplied to it or terrarium/deoxider near the area where most dupes are. So this usually tends to be by the printing pod, either one same level or below. The other tends to be opposite side of it higher up. I watch the oxygen overlay a lot, so any areas that are low pressure I try to either block them off if I'm not using them so less room for gas to spread out, or I make another vent/algae deo there.

I've had success with algae terrariums patches ago where having a bunch at the bottom of my base would overpressurize it to ridiculous amounts and watering them with a liquid vent, but honestly its a waste of water.

Basically you have a couple of options if you can't build an electrolyzer yet. Algae Deoxydizer, one for every 5-6 dupes, I think 5 is a safe number, if you have 1 with divers lungs you'll eventually have pressurized base. Algae terrariums are good for mining far away from home but I wouldn't use them since they still delete water pretty much. Polluted oxygen is safe as long as you get rid of germs. Speaking of which, I've been a fan of air deodorizers. I put a couple in my piss pool room with compost in case of rotted food. While it's not a lot, it's another production, same with putting them in a biome that has polluted water. 80kg sand for enough air for a dupe or two to mine there.

Something more advanced would be converting polluted oxygen directly to oxygen via temperature, making it really cold. I would recommend brothgar's videos and the ONI forum. Plenty of interesting ideas and builds there. Basically, take some PO2 source like morbs or pee pee water air and make it cold enough to force a state change from gas to liquid with a trough/pathway for it to become a gas again but O2 instead of PO2. Or you can pump it while its still liquid oxygen. Or you could do other things with it.

You could also turn your polluted water into regular water, either heat it up so it becomes steam then collect the water afterwards. I think dirt is also a byproduct, but I haven't messed around with heating stuff up in a long long time. Or use the water purification machine, but I've never used this, I don't think I've even built one ever.

I think it always comes down to the point in every game you'll just end up with lots of polluted water. If you're low on water and can't find a geyser and your algae supply is running low, consider one of the above.

Here's an example of my current base. I have a steam geyser that I found on like cycle 3 and a natural gas geyser to the bottom left I found on cycle 5ish. I just started using the geyser water around cycle 80is when I stopped getting more dupes at 12 and I haven't even bothered with the natural gas geyser as I've been exploring the map.