The estimation of the ocean heat content (OHC) is based on the subsurface temperature observations which abundance, quality and accuracy changed over time following the development of oceanographic techniques and methods. The majority of the available global OHC time series begins in 1950s when the International Geophysical Year 1957–1958 (IGY) observational campaign resulted in a rapid improvement of the observational basis. However, a significant amount of subsurface temperature observations was accumulated during the period before the IGY. We perform a three-step quality control (QC) of the Nansen cast temperature profiles with the focus on the time period before 1970. The QC includes automated and manual control of temperature observations along with the quality control of temperature anomalies. The World Ocean Circulation Experiment/Argo Global Hydrographic climatology based on the high-quality temperature observations between 1990 and 2025 is used as the reference. To interpolate temperatures vertically we use a combination of the parabolic Reiniger and Ross (1966) method and the temperature anomaly method by Frankignoul (1981). The method is shown to result in a smaller interpolation error. Correction for depth bias in Nansen cast profiles are also suggested. The oldest temperature profiles from 1870s are treated separately, with the data from “Challenger” expedition used as reference. Finally, we estimate the OHC changes between 1870s and 2020s for six selected multi-year time periods. We find that for the layer 0–2000 m the OHC increase between 1920–1944 and 1945–1969 amounts to 30 % of the total OHC increase between 1920–1944 and 2010–2025.