The relationship between osteoarthritis (OA) and osteoporosis (OP), the two most common skeletal disorders related to aging, is controversial. Previous studies suggest that OA is inversely related to OP when studied cross-sectionally and systematically. However, there are differences in the results depending on the parameter used to define OA. The purpose of this review is to analyze and summarize the literature, and derive possible answers to three key questions along with a brief introduction on underlying mechanisms: (1) Is OA correlated to a high bone mineral density (BMD)? (2) Does OA influence the progression of OP or osteoporotic fractures? (3) Does high BMD affect the incidence and progression of OA? A review of the literature suggests that OA is inversely related to OP in general when studied cross-sectionally and systematically. However, when analyzed in individual bones, the BMD of the appendicular skeleton in OA-affected joints may decrease, particularly in the upper extremities. On whether OA influences bone loss or osteoporotic fractures, differences are observed according to the affected joints. The risk for osteoporotic fracture does not seem to decrease despite a high BMD in patients with OA, probably due to postural instability and muscle strength. Low BMD at the lumbar spine is associated with a lower incidence of knee OA although it does not arrest the progression of knee OA.