From The Document Foundation Wiki
How do I draw an ellipse from its center?
- Select the ellipse drawing tool;
- Hold down the Alt key;
- Click and drag to draw the figure. The ellipse or circle uses the start point (where you first clicked the mouse button) as the center.
This works the same way for other shapes (rectangle, star, ...). The Alt key can be combined with the ⇧ Shift key to draw a symmetrical figure.