Put your formula: =SUM( O2:O48) into cell P48.
Select cells P2:P48 as a single block.
Without de-selecting that range grab the drag-handle at the bottom left of cell P48 with the mouse and drag down as far as you need.
You may have to adjust this a bit because your first sum sums 47 rows, your second one 49 rows and your third 48 rows.