  The `O-rich AGB sequence' is a sequence of colours describing the location of
O-rich AGB stars in the IRAS two-colour diagram [12]--[25] vs [25]--[60]. We
propose an evolutionary scenario for this sequence in which all stars,
independent of their progenitor mass, start the AGB phase in the blue part of
the `O-rich AGB sequence' and then evolve toward redder colors, although only
the more massive stars would reach the very end of the `O-rich AGB sequence'.
The sources located in the blue part of the sequence are mainly Mira variables,
whose mean period is increasing with the IRAS colours. Most of them will evolve
into O-rich Type II (and III) Planetary Nebulae. Part of the stars located in
the red part of the sequence will change their chemical composition from O-rich
to C-rich during their evolution in the AGB phase, and might evolve into C-rich
Type II Planetary Nebulae. Hot bottom burning may prevent the conversion to
carbon stars of the rest of sources located in the red part of the sequence and
they will end up as N-rich Type I Planetary Nebulae.
