  Noticing the potential relationship between the pulsation amplitude of
fundamental mode RR Lyrae stars and their average effective temperatures, we
derived the relation between them from observational data. Using these
relationships, we present the zero-age horizontal branch lines and
corresponding evolutionary tracks in the Bailey diagrams. We first find that
the trends of zero-age horizontal branch lines are consistent with the
distributions of Oosterhoff I RR Lyrae stars, and there is a period shift
effect in relatively metal-rich samples ([Fe/H] > -1). Main results show that
evolutionary effects are the direct cause of the Oosterhoff dichotomy. The vast
majority of Oosterhoff I RR Lyrae stars are in the early stage of horizontal
branch evolution, while most of the Oosterhoff II stars are near the end of the
horizontal branch phase. Our Bailey diagrams can intuitively explain some
Oosterhoff phenomena (e.g., long-period RR Lyrae stars in the metal-rich
globular clusters NGC 6388 and NGC 6441), and also have the potential to become
an important tool for in-depth study of RR Lyrae stars. Further research is
needed on the Oosterhoff dichotomy to explore the formations and evolutionary
processes of the Milky Way and its substructures.
