The current study presents new bed-by-bed brachiopod δ¹³C and δ¹⁸O records from Öland, Sweden, which together with previously published data from the East Baltic region, constitutes a high-resolution paired brachiopod and bulk rock carbon and oxygen isotope archive through the Lower to Upper Ordovician successions of Baltoscandia. This new data set refines the temporal control on the global Ordovician δ¹⁸O-trend considerably, improving paleoenvironmental reconstructions through the main phase of the Great Ordovician Biodiversification Event (GOBE). The new brachiopod carbon and oxygen isotope records from Öland display strong similarity with the East Baltic records, elucidating the regional consistency as well as global correlation utility of the ensuing composite Baltoscandian Lower to Middle Ordovician carbon and oxygen isotope record. The carbon isotope record from Öland indicates that the widely reported Middle Ordovician carbon cycle perturbation—MDICE (Mid-Darriwilian Carbon Isotope Excursion)—is recorded in both brachiopods and bulk carbonates. The oxygen isotope record reveals a long-term Lower to Upper Ordovician trend of increasingly heavier brachiopod δ¹⁸O values, with a pronounced increase during the Middle Ordovician Darriwilian Stage. We interpret this trend as dominantly reflecting a paleotemperature signal indicating progressively cooler Early to Middle Ordovician climate with glacio-eustasy. Our Baltic δ¹⁸O values are therefore consistent with postulations that the biotic radiations during the GOBE and climatic cooling during the Darriwilian were strongly linked.