I want to have the date entered in cell O3 when Completed is chosen

Thank you for your reply
I need it to go down the whole collum and i need it to enter the date in the corresponding row
E.g when completed is selected in M3, cell O3 shows the date that completed was selected
this goes down the sheet so that everytime cell M? has completed the corresponding cell row in O shows the date that completed was selected
Cheers