Summary The effects of oleuropein on the processes of osteoblastogenesis and adipogenesis in mesenchymal stem cells (MSCs) from human bone marrow have been studied. We report that oleuropein, a polyphenol abundant in olive tree products, reduces the expression of peroxisome proliferator-activated receptor gamma (PPARγ), inhibits adipocyte differentiation, and enhances differentiation into osteoblast. Introduction Age-related bone loss is associated with osteoblast insufficiency during continuous bone remodeling. It has been suggested that the formation of osteoblasts in bone marrow is closely associated with adipogenesis, and age-related changes in this relationship could be responsible for the progressive adiposity of bone marrow which occurs with osteoporosis. In addition, the consumption of oleuropein, a major polyphenol in olive leaves and olive oil, has been associated with a reduction in bone loss. Methods We have analyzed the effects of oleuropein—at concentrations between 10−6 and 10−4 M—on the processes of osteoblastogenesis and adipogenesis in MSCs from human bone marrow. Results The results show an increase in osteoblast differentiation and a decrease in adipocyte differentiation when there is oleuropein in the culture media. The gene expression of osteoblastogenesis markers, RUNXII, osterix, collagen type I, osteocalcin, or alkaline phosphatase (ALP), was higher in osteoblast-induced oleuropein-treated cells. Also, the ALP activity and extracellular matrix mineralization were higher when oleuropein was present in the media. Oleuropein in MSCs induced adipocytes to produce a decrease in the expression of the genes involved in adipogenesis, the PPARγ, lipoprotein lipase, or fatty acid-binding protein 4, and minor fat accumulation. Conclusion Our data suggest that oleuropein, highly abundant in olive tree products included in the traditional Mediterranean diet, could prevent age-related bone loss and osteoporosis.