Is it necessary to add +0.5 to the oral temperature ? and when to add 0.5 and when to substract ?

Not necessary: Although oral temperatures tend to be lower than rectal temperatures (the most accurate measure of internal body temperature) by about 0.5 degrees f, it's still roughly within measurement error and doctors won't expect you to be doing math. It's always best to state the true measurement and whether it was oral, axillary (armpit), rectal, or tympanic (ear).