The essential oil coming from the rind of the orange (the sweet one). In general, the main component of citrus peel oils is limonene (83-97% for sweet orange peel), a super common fragrant ingredient that makes everything smell nice (but counts as a frequent skin sensitizer).
Other than that, citrus peel also contains the problematic compound called furanocoumarin that makes them mildly phototoxic. Orange peel contains less of it than some other citruses (like bergamot or lime), but still, be careful with it especially if it is in a product for daytime use.
Show me some proof
- Shaw, Philip E. "Review of quantitative analyses of citrus essential oils." Journal of Agricultural and Food Chemistry 27.2 (1979): 246-257.
- Ahmad, Muhammad Mushtaq, et al. "Genetic variability to essential oil composition in four citrus fruit species." Pakistan Journal of Botany 38.2 (2006): 319.
- Nathalie, Dijoux, et al. "Assessment of the phototoxic hazard of some essential oils using modified 3T3 neutral red uptake assay." Toxicology in vitro 20.4 (2006): 480-489.
- Dugrand, Audray, et al. "Coumarin and furanocoumarin quantitation in citrus peel via ultraperformance liquid chromatography coupled with mass spectrometry (UPLC-MS)." Journal of agricultural and food chemistry 61.45 (2013): 10677-10684.