Some oils are better for your hair than others.  Examples of oils that can be used on your hair are jojoba, coconut and olive oil.  Not all hair types like all types of oil.  My fine hair prefers jojoba oil and will be greasy looking with coconut or olive oil, but many who have curly or coarser hair prefer the olive oil.

Hot oil treatments are really WARM oil treatments, you do not want the oil to be too hot or you could damage your hair and/or your scalp.  The best way to do an oil treatment is to dampen your hair then apply the warm oil, concentrating on the ends and length of the hair and less on the hair on your scalp.  The oil can be warmed by placing a container with the oil in hot water for about 5 minutes.  Once the oil is on the hair, wrap your hair around your head (if long enough), then wrap your head with plastic wrap (optional) and a warm towel.  When the towel has cooled, you can replace it with another warm towel if you want.  Wash and condition your hair.

You can extend the treatment by oiling your hair before bed and washing it in the morning.  The heat from your head will help the oil to be absorbed by the hair shaft.

I hope this helps!