Although the importance of stable isotope ratios in tree rings is increasing for high-resolution climate reconstructions, it is still unclear if such values exhibit age trends that require some form of standardization. Here we present 13,496 and 13,584 annually resolved and absolutely dated δ¹⁸O and δ¹³C measurements from 147 living and relict oaks (Quercus spp.) that grew over the past 2,000 years in the Czech Republic. In contrast to their heteroscedastic ring widths, the stable isotopes reveal constant spread versus level relationships over the trees' life span. Together with high signal strength, the absence of age-related constraints makes δ¹⁸O and δ¹³C from oak latewood alpha cellulose a superior climate proxy in regions where traditional tree-ring parameters are limited.