When you need to use 2 or more columns in a spreadsheet to display a title then you need to merge the cells, otherwise the title will hide below the next cell. The difference is that merged cells behave as one cell. So for example if you want to make a table of data and you want to have a centered title above, you can only accomplish that by merging the cells above the data which holds the title.
To merge cells in an OpenOffice.org spreadsheet you must first select the cells you which to merge.
And then click at the menu Format -> Merge Cells (you can see it at the image)
If you want now to split a cell into the normal columns (as it was in the beginning) then repeat the above merging procedure.
Note: The procedure of cell merging is not the same at other spreadsheet software.