The final stages of low-mass stellar evolution are characterized by significant mass loss due to stellar pulsations during the AGB phase, which lead to the development of planetary nebulae. Molecular masers of H2O, SiO, and ground state OH transitions are commonly detected in oxygen-rich late-type stars (OH/IR objects). In contrast, excited OH maser transitions are rare. We discuss our study of the carbon-rich pre-planetary nebula CRL618 (a prototypical post-AGB star). Observations conducted in May 2008 with the 305m Arecibo Telescope resulted in the first detection of a 4765MHz OH maser line in a late-type stellar object; the detection was confirmed a few months later also with Arecibo. Subsequent observations in 2015 and 2017 resulted in non-detection of the 4765MHz OH line. Our observations indicate that the 4765MHz OH maser in CRL 618 is highly variable, possibly tracing a short-lived phenomenon during the development of a pre-planetary nebula.