In your open document in OpenOffice.org:

Open the Stylist (Format-Stylist or [F11])
In the Stylist window, click on the Page Styles icon (fourth icon from left)
The type "Standard" should already be highlighted, hold the right menu button down to see the context menu and choose "New..."
In the dialog that appears, give the new page style a descriptive name, e.g. Landscape
Click on the [Page] taband change the page orientation to Landscape by clicking on the corresponding radio button. You may also make other changes to the page style, if needed (such as adjusting the margin sizes).
Click on [OK]. The new style will appear as one of your page style choices.
Place the text cursor in your document at the location where you want to insert the Landscape page
Select Insert-Manual Break
In the dialog that appears, select Page Break and in the Style list, select your new page style, e.g. "Landscape";
Click on [OK]
.

To end the Landscape area in your document:

Place the text cursor in your document at the location where you want to stop the Landscape layout
Select Insert-Manual Break
In the dialog that appears, select Page Break and in the Style list, select the previous page style, e.g. "Standard"
Click on [OK]