Understanding the origin of ocean island basalts (OIB) has important bearings on Earth’s deep mantle. Although it is widely accepted that subducted oceanic crust, as a consequence of plate tectonics, contributes material to OIB’s formation, its exact fraction in OIB’s mantle source remains ambiguous largely due to uncertainties associated with existing geochemical proxies. Here we show, through theoretical calculation, that unlike many known proxies, triple oxygen isotope compositions (i.e. D17O) in olivine samples are not affected by crystallization and partial melting. This unique feature, therefore, allows olivine D17O values to identify subducted oceanic crusts in OIB’s mantle source. Furthermore, the fractions of subducted ocean sediments and hydrothermally altered oceanic crust in OIB’s mantle source can be quantified using their characteristic D17O values. Based on published D17O data, we estimated the fraction of subducted oceanic crust to be as high as 22.3% in certain OIB, but the affected region in the respective mantle plume is likely to be limited.