Urban heat islands (UHIs) in the built environment are becoming alarmingly severe, calling for urgent climate change mitigation measures. Common data collection and data-driven analysis protocols are needed to efficiently study this phenomenon across different urban areas. The emerging low-cost sensing systems and weather data crowdsourcing are showing a great potential to support these efforts. This study explores the spatial patterns of UHIs in the city of Oslo, Norway, as similar studies in northern European cities are still too scarce. It leverages the available weather data from municipal stations and crowdsourced weather observations with high spatial resolution (over 2800 stations in the area) to characterize the phenomenon at city scale. The results show that Oslo is affected by UHIs, with its intensity reaching up to 5.5° in some districts. They also suggest that in particular conditions the spatial distribution of UHIs does not always follow the typical pattern, i.e., higher temperatures in downtown and more urbanized areas, and lower temperatures in less dense areas with more open and green spaces. Occasional negative UHIs intensity values were also captured. These effects may be the result of the strong influence of weather phenomena and topography. The findings in this study indicate the need for careful consideration of the underlying local conditions to better understand UHIs drivers and develop more targeted urban planning policies and mitigation strategies. Therefore, future studies should delve into the links between UHIs intensity and urban form characteristics.