Well warm to hot.
On a few occasions, i have had the great pleasure of having a lady perform OWO while she has warm/hot water in her mouth.
Another time it was menthol mouthwash.
These produce the most wonderful sensations.
I have also heard of Alka Seltzer, or Poppin candy being used.
I have a question for the ladies, would this be equally as nice if it was done to you?