VBA Filldown an IF-Then Statement

I am an amatuer self-taught programmer. I use the fill down function frequently in my spreadsheets and I thought I got the concept down fairly well. I can't seem to figure out how to filldown this IF-Then statement. I get it to run now on the first line but I need it to filldown the statement to the last row in the sheet which may vary sheet to sheet.

This is the statement I have.

If Range("A17").Value="D01" Then
    Range("S17").Value="I"
ElseIf Range ("A17").Value="D02" Then
    Range("S17").Value="O"
End If