The isotopic compositions of tracers such as O and Si in silicified sedimentary rocks have been used as proxies to reconstruct oceanic temperatures throughout the Earth’s history. However, great controversies still remain, notably regarding possible variations of the O isotopic composition of the ocean through time. Direct O isotope analysis of kerogens preserved in Archean cherts have the potential to provide crucial insights to this ongoing debate, but such measurements have not been carried out so far. During this seminar I will present the analytical procedure we have developed for in situ measurement of the O isotope composition of fossil organic matter by secondary ion mass spectrometry at the sub-per mill level, and the results obtained on both Phanerozoic coal and kerogen samples. Preliminary results obtained on Archean kerogens ranging in age between ca. 3.0 Ga and 3.4 Ga suggest that the O isotope composition of Archean oceans was not drastically different compared to the present-day composition, suggesting, therefore, that oceanic temperatures have continuously decreased throughout the Earth's history.