Why does my freshly painted bedroom still smell like a litter box?

Check the unsealed edges of the bedroom door and the window casing before you give up. Most painters skip the top and bottom edges of doors because nobody sees them, but that raw wood is extremely porous and holds onto odors for years. If the previous tenants were as gross as you say, that door is essentially a giant scent sponge. You should actualy take the door off the hinges and sniff the bottom edge; if it reeks, that’s your source. You'd be surprised how much a hollow-core door can store and then slowly vent back into a room.

The window casing is the other likely spot. If the smell is strongest in that back corner, it's probaly because the air pressure is drawing the scent out from behind the trim. In many rentals, the gap between the window frame and the wall is stuffed with old insulation that has absorbed every nasty smell the house has ever seen. Since you can't rip the trim off, your best move is to seal every posible air gap. Ozone machines are great for surface smells, but they won't touch anything trapped behind a barrier. If the odor is coming from the subfloor or from behind the baseboards, the ozone simply can't reach the source to neutralize it. When maintenance says a floor was 'treated,' it ussually means they just slapped a single coat of cheap primer down, which is rarely enough to stop urine off-gassing. The scent is likely venting out from the expansion gap where the new vinyl meets the wall. Try using a **shellac-based primer** on the unpainted top and bottom edges of the door and then use a high-quality caulk to seal the baseboards to the floor and the wall. You want to make that corner as airtight as possible so the subfloor can't 'breathe' into your living space. It’s a relatively cheap, non-invasive way to mask the issue without needing to rip into the structural parts of the rental.

I never thought about the top and bottom of the door being unpainted. I'm a total newbie at DIY stuff, but could the smell also be stuck in the hinge pins or behind the metal plates? I noticed on my own bedroom door that there's alot of old grease and gunk in the hinges that smells kind of funky. If the previous people were realy messy, maybe some of that 'scent' got into the actual hardware? I wonder if swapping the hinges and the knob woud help since those are easy to change without the landlord getting mad.

Check the door knob and the hole it sits in. If it’s a hollow-core door, the inside is a honeycomb of cardboard that acts like a massive sponge for odors. If the previous tenants were as bad as you say, that air is stuck inside the door shell. When the room heats up or the pressure shifts, the door 'exhales' that stank. Pop the knob off and sniff the interior. If it's the source, you'll need a new door slab. No amount of scrubbing the outside will fix a smell that's literally trapped inside the door itself.