Excel 2007 : Basic formula question for Excel 2007 --copying same formula to apply to whole column

I know this is a basic question, but I can't figure it out even after looking at self-help books on the subject:

Let me just give a specific example.

I placed a formula in p3 that adds D3:O3. This works fine on line 3, that is the first line where the formula is.

But how can i also implant this same formula in the cell right below O3, that is O4, O5, etc. to duplicate this same addition in all the lines. I know this can be done without individually doing this for every line, but for some reason I have a mental block about this.

Thanks very much for helping me solve this issue.

Carroll Iorg