The basic theory of spirals is basically that you get spiral structure when the orientation of elliptical orbits shifts with semi-major axis. This leads to what appear to be spiral arms that have higher densities. In these high density regions you get collisions of gas clouds which leads to star formation. The star forming regions produce lots of bright, blue stars, which then make the spiral arms very visible in optical wavelengths.

In this case of the Oort cloud the galactic tides would be what are responsible for inducing the change in orientation of the elliptical orbits as a function of semi-major axis.