Diagnosis
The anti-reflective coating on LG OLEDs is notoriously delicate. Dry dusting or using a dry microfiber on fingerprints can scratch the coating or just push the oils around, causing smears.
How to Fix It
Use distilled water only. Never use window cleaner, alcohol, or any chemical solvents. Lightly dampen a clean, soft microfiber cloth with distilled water, wring it out completely so it's just barely damp, and gently wipe the screen in straight lines, not circles. Let it air dry. For stubborn spots, you can breathe on the screen to add a tiny bit more moisture to the cloth.
If That Didn't Work
LG sells an official screen cleaning kit (model SA-CK1A) that's formulated to be safe for their anti-reflective coating. If you're really worried, that's the guaranteed-safe option. Aftermarket screen cleaning solutions are a huge risk.
Heads Up
Never spray any liquid directly onto the screen. It can run down into the ultra-thin seams at the edges and damage the electronics. Always apply the liquid to the cloth first. And press gently—you're wiping a film of oil, not scrubbing a stain.