The use of Olive-oil have been prescribed by several doctors, it makes the hardened ear-wax softer, and loosen it from the ear-channel walls so the deposits comes out easy when using the rubber-syringe to flush the ear-channel with luke-warm water.
The rubber-ball with a soft syringe/tube you get at the pharmacy.
The oil also softens the skin, and relieves the irritation the skin is subjected to when cleaning your ears.
You do not need to have the oil in there all night, only keep it in there 30 mins/one hour, that is sufficient.
I use soft ear-plugs to seal the ear and keep the oil in there when I do this procedure.
Any other stuff, like Q-tips and cotton-balls is not adviced by any doctors.
Plus, considering that we humans spend our entire lives completely covered in a thin coat of oil/grease secreted by our skin and we manage to usually not rot until we die... olive oil is probably not going to rot in your ears.
The Wikipedia article, though, does mention that there's very, very little difference in effectiveness between using oil-based solutions (like straight olive oil or Debrox) and using just plain ol' water. So if you're queasy about pouring oil into your ears, apparently it's only a miniscule bit more effective than using water anyways.