Is there an exact torque value that should be used to tighten the propeller nut? should this nut have locking threads? Are there different torque values for different engines? How about specific size propellers for specific planes? Any help would be greatly appreciated.

No idea, just firm enough, no diferant to any other bolt or nut of equvalent size

It needs to be tight. Use at least a 6" wrench and don't be afraid to put some pressure on it. I've seen many a prop come loose because they simply weren't tight enough.

I double-nut my OS-48. Helps prevent loosening on kickback.

On that size thread it would normally be a maximum of about 9 footpounds. I've never had a prop come loose so one day I checked how tight I do them up. I tightened it the normal way then used a torque wrench. The breakaway torque was about 5 footpounds (or 60 inchpounds if you prefer).

About 3 grunts lol.

I like to use nylon lock nuts on mine for added portection, as far as torque just get it good and tight!

I learned a scary lesson on tightening prop nuts today. I dont know even know how it came loose, but I fired it up and after about a second of WOT it flew off and nearly hit my dad. Scary stuff, well now its incredibly tight, it might've come loose from oil on the crankshaft, does anyone recommend putting locktite on this thing?

This is why you should NEVER be in the same plane as the spinning prop and make sure that NO OTHER PILOTS are either. I see guys running their engines at full throttle while in front of the plane AND reaching over to hold it down, NOT a good idea! Don't use locktite on the prop shaft, it will only complicate things as you try to change props or check for tightness. Use a long enough wrench to get some leverage on the nut and while holding the prop in the other hand crank that thing tight. Wood props will tend to compress when they're new so they need to be checked more frequently. If you ever have the least bit of doubt that the prop is not tight enough, by all means check it before you start it. 12,000+ RPM's on a 1oz piece of nylon or wood that decides to part company is lethal to say the least. Let's be as safe as we can, most accidents can be avoided.
By the way, any oil on the prop shaft is not a good thing. If there's oil there, clean it off. Stay safe guys!

I've cleaned it before and it keeps getting oiled. What could this be?

ALL NUTS ARE NOT CREATED EQUAL!
I tightened mine several times.
I then got close to 4 foot pounds and I could feel the threads giving.
Can't beat a nylock nut.
Also in a real pinch you can smack that nut 1 time with a hammer on appx a 30* angle.
It will deform the nut and it will start fine then get tight about half way.
Some non Nylon lock nuts are actually made that way

Stick Jammer.
I also see this very often
A long ride to the hospital while that severed artery is held tight places a load on the heart.If a freak accident like that did happen assuming it would take 5 seconds for some fast thinking person to try to stop the bleeding the injured person has already lost close to 2 pints of blood.
Very bad practice indeed