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. Plain Language Summary Oxygen isotope values obtained from fossil brachiopod shells have traditionally been used as a faithful paleoclimatic proxy to shed light on temperature trends in ancient oceans. However, because brachiopod shells are susceptible to diagenetic overprint after burial, secular oxygen isotope trends derived from these fossils are often questioned—notably the farther one goes back in geological time. This study presents temporally well‐resolved oxygen isotope data from Lower–lower Upper Ordovician sedimentary rocks of Öland, Sweden, tied precisely to conodont biostratigraphy on the bed‐by‐bed scale. This interval is important in Earth history as it brackets the greatest marine biodiversification event known in the fossil record and coincides with a global climatic cooling phase (determined based on proxies other than oxygen isotopes). The current study therefore provides an excellent test of the spatial and temporal consistency of the secular Ordovician oxygen isotope trend. We find that although our data is probably affected by diagenetic modification, primary paleoclimatic signals are preserved. Furthermore, as current global Ordovician oxygen isotope records lack sufficient resolution because they comprise data from geographically widely distributed low‐paleolatitude localities, our new high‐resolution data set from one mid‐paleolatitude region, provides significant temporal insights that considerably improves our understanding of the Ordovician climate. Key Points New paired Baltic carbonate data set improves Ordovician 18O‐ and 13C‐record The new data supports a Middle Ordovician sea surface temperature cooling Regional/intra‐basinal consistency of oxygen isotope trends indicate primary nature of paleoenvironmental changes