I don't think all of the symptoms are related, unless you have a blown headgasket, which could account for two of them - the oil leak/coolant leak. What you have are spurious correlations - things that seem interrelated but are not.
1) Coolant Issue - low coolant means no heat. I would venture to say you have either a stuck thermostat, a crumbling coolant reservoir, leaking hoses, a bad waterpump or a gunked up radiator. Or all of the above.
2) Low oil - I've been searching these boards for possible oil leak locations. Depends on which side of the car you find the oil puddled. Do a search once you locate the oil field.
3) Defrost/AC - possibly in the climate control panel? First, check your dash vent - is the adjuster on blue for cold or red or heat. If it is on red, that will make A/C warm. Check your freon level, too.
Hook the car up to a code reader and see what's doing. Might answer some questions for you.