In OO, headers are specified by page styles.
- Open the Page Style dialog box for the desired page style (right-click > Modify).
- Select the Header tab.
- Under Header, check Header on and click OK.
The simplest case is to have the page number at the top of every page and nothing more. To do this, put the cursor on the header and select Insert > Fields > Page Number.
Presto! Now, the correct page number appears on every page.
Find more information on this feature here.