From The Document Foundation Wiki
How do I draw a circle or a perfect square?
- Select the Ellipse icon or the Rectangle icon in the Drawing toolbar;
- hold down the ⇧ Shift key;
- Click and drag to draw the shape.
This works the same way for the other shapes (star, diamond, etc.). The ⇧ Shift key can be combined with the Alt key to draw a figure from its center.