  We present a streamlined approach to relative oscillation criteria based on
effective Pruefer angles adapted to the use at the edges of the essential
spectrum.
  Based on this we provided a new scale of oscillation criteria for general
Sturm-Liouville operators which answer the question whether a perturbation
inserts a finite or an infinite number of eigenvalues into an essential
spectral gap. As a special case we recover the Gesztesy-Uenal criterion (which
works below the spectrum and contains classical criteria by Kneser, Hartman,
Hille, and Weber) and the well-known results by Rofe-Beketov including the
extensions by Schmidt.
