The flora of Oasis de los Osos is an unusual assemblage of species drawn from the surrounding mountains and deserts of Southern California. The 160 acre palm oasis is a University of California Natural Reserve administered by the University of California, Riverside. The Oasis is located in the western Coachella Valley at the northwestern edge of the Sonoran Desert and on the northern slope of the San Jacinto Mountains. Botanists have visited the area since the 1890s, facilitated by the railroad route through San Gorgonio Pass, and have been intrigued by its location in a transitional zone between Sonoran Desert, Mojave Desert, montane, and South Coast floristic regions. Since the 1970s, the Oasis has been protected from development, first by the Nature Conservancy and then as a University of California Natural Reserve. Despite this long history of botanical collections and protection, there has never been a checklist published for the area. We made a comprehensive list of the vascular plants present at the Oasis by documenting the flora with our own botanical surveys and with historical collections in herbaria. We annotated the checklist with information on nativity, habit, rarity, and distribution. A total of 164 native and 24 non-native taxa were documented from the Oasis, including one rare plant, Galium angustifolium subsp. gracillimum (California Rare Plant Rank 4.2). The vegetation in the Oasis transitions from brittlebush scrub at low elevations to mixed scrub at higher elevations. Shrubs are the dominant cover type and represent 20% (38 taxa) of the plant diversity. The riparian habitat is fed by Lamb Creek, a stream that supports native fan palms, willows, cottonwoods, and a variety of herbaceous species. The Oasis flora represents an unusual mixture of taxa from the surrounding San Jacinto and San Bernardino mountains (138 taxa, 73%) and the Sonoran and Mojave Deserts (109 taxa, 58%). Invasive grasses such as brome (Bromus spp.) and Mediterranean grass (Schismus barbatus) are the dominant ground cover at lower elevations, but the other 14 invasive species we documented are not well- established. The Oasis contains a great deal of plant diversity, considering its small geographic area. We compared the Oasis flora with Deep Canyon and found that 67% of Oasis species are also present at Deep Canyon. The Oasis de los Osos is a largely intact palm oasis and canyon that is representative of the transitional floristic diversity of the western Coachella Valley.