What is the purpose of the oval office?

It's oval because that way the president can hear everything.

There are two points in an oval (or ellipse) called the focus point.

In a room such as the oval office, when standing at one of these points you can hear everything that is happening in the room because sound is bounced off the walls/ceiling sending it all to those points (basic geometry).

The president's desk is situated on one of those points meaning the president can hear whatever anyone is saying (even when whispering) so you can't "keep a secret" from the president in his office.

The mason's designed it.

Hope this helps