In this study, the oxygen isotope (O-18) composition of pink to red gem-quality rubies from Paranesti, Greece was investigated using in-situ secondary ionization mass spectrometry (SIMS) and laser-fluorination techniques. Paranesti rubies have a narrow range of O-18 values between similar to 0 and +1 parts per thousand and represent one of only a few cases worldwide where O-18 signatures can be used to distinguish them from other localities. SIMS analyses from this study and previous work by the authors suggests that the rubies formed under metamorphic/metasomatic conditions involving deeply penetrating meteoric waters along major crustal structures associated with the Nestos Shear Zone. SIMS analyses also revealed slight variations in O-18 composition for two outcrops located just similar to 500 m apart: PAR-1 with a mean value of 1.0 parts per thousand +/- 0.42 parts per thousand and PAR-5 with a mean value of 0.14 parts per thousand +/- 0.24 parts per thousand. This work adds to the growing use of in-situ methods to determine the origin of gem-quality corundum and re-confirms its usefulness in geographic "fingerprinting".