This is the oxygen setup I've been using:
(tho I always pipe the hydrogen thru a filter to avoid damaging my generator. same for oxygen, but only before it goes into exosuits).
This plus one water pump, it just barely uses under 1000 power.
As soon as I've got some basic necessities running (farms, better batteries, light for my bristles), I start researching what I need to build this, including the smelter thingy for the automation wires. I always only smelt gold, so I try to find a place where I can tunnel to some without disturbing any slime blocks - mining slimes releases the slimelung. Usually even the swampy biomes are not hazardous before you mine at least one slime. There are exceptions.
If you managed to mine slime, try to drown it in some liquid so it doesn't release polluted oxygen with the germs.
Then it's really important to get insulated tiles. I surround my base with a insulator box (avoid digging to much towards the granite, to keep the heat away until you can do this).
I keep the heat producing stuff on the outside, that included my electrolyzers, batteries, and my food grills - the latter are just outside the insulated area, easy to access. Once I get exosuits, the only way to exit the insulated box is thru a checkpoint.
I don't know if this is still the case, but electrolyzers always output hot oxygen, so if you have cold water, maybe pipe it thru somewhere hot that you want to cool down (maybe using radiant pipes, or to feed a smelter) before inputting it into the machine.
EDIT: Forgot to add, since oxygen is always outputted at 60-70°C, don't pipe it straight into your base, let it spread the heat a bit outside your cooler area with a combination of radiant and insulated pipes - even if the air is 40°C outside your base that's still better than 70°C.