Shale oil is a promising alternative unconventional energy to conventional fossil fuels. Its exploration and production are critical to improve the present energy consumption pattern and mitigate dependence on foreign energy. Particularly, the key factors determining shale oil occurrence are the most important issues for its production. In this review, we briefly summarize the dominant space of shale reservoirs for accommodating shale oil. We also provide a comprehensive overview of occurrence state of shale oil and analyze in detail the factors such as pore size, organic matter, inorganic minerals of oil-bearing shale and reservoir burial depth. Finally, we discuss the existing challenges and perspectives on how to explore and address occurrence of shale oil in practical shale reservoirs to promote the future shale oil production. An accurate description of shale oil occurrence through viable research methodology, advanced instrument characterization and theoretical simulation is crucial for estimating shale oil reserves, optimizing and enhancing shale oil production.