Galactic Oscillation Frequency question....?

Let's start with a couple of statements - there's a huge cloud of comets orbiting the sun, way out beyond the orbit of Pluto, called the Oort cloud. Our galaxy is basically disk shaped, rather like 2 fried eggs back to back - the plane of the galaxy is where the two eggs touch and it's theorised that there's more interstellar material there.

The sun orbits the centre of the galaxy, but its orbit is at a slight angle to the plane of the galaxy. This means that half the time it's above the plane of the galaxy and half the time it's below it. Obviously it has to cross the plane of the galaxy to do this, and that means it passes through the part where the density is higher. The theory is that the gravity of this extra "stuff" alters the orbits of comets in the Oort cloud and some of them (more than usual) fall in towards the inner solar system. With a higher than usual number of comets coming our way there's a greater chance that we'll be hit by one. Some people believe there's a correlation between the frequency of mass extinction events on Earth and the solar system crossing the galactic plane.