Measurements of the O-16 and O-18 distribution functions in the solar
wind at low to average solar wind speeds from the MASS instrument on t
he wind spacecraft are reported. The O-16/O-18 density ratio is 450 +/
- 130, a value consistent with terrestrial, solar photospheric, solar
energetic particle, and galactic cosmic ray O-16/O-18 isotopic ratios.
This study constitutes the first reported spacecraft measurement of t
he isotope O in the core solar wind and may represent the best determi
nation of the solar O-16/O-18 density ratio to date.