Osteosarcoma (OS) is a lethal bone malignancy. Circular RNAs (circRNAs) have emerged as important regulators of OS development. CircRNA cyclin dependent kinase 14 (circ_CDK14) was reported to be a potential oncogene in OS. However, the mechanistic pathway by which circ_CDK14 functions in OS is largely unknown. The relative expression of circ_CDK14, microRNA (miR)-520a-3p, and GRB2 Associated Binding Protein 1 (GAB1) was evaluated by quantitative real-time PCR and western blot assays. Flow cytometry was employed to monitor cell cycle distribution and apoptosis. Methyl thiazolyl tetrazolium (MTT) and colony formation assays were performed to assess cell viability and colony formation ability, respectively. Western blot assay was also used to detect the expression of apoptosis-related proteins. Transwell assay was carried out to monitor cell migration and invasion. Additionally, the target association between miR-520a-3p and circ_CDK14 or GAB1 was confirmed by a dual-luciferase reporter assay. Xenograft assay was applied to investigate the role of circ_CDK14 in vivo. Circ_CDK14 and GAB1 expression was upregulated, while miR-520a-3p was downregulated in OS tissues and cells. Circ_CDK14 depletion hindered OS cell proliferation, metastasis, and tumorigenesis while facilitated apoptosis, which were all ameliorated by miR-520a-3p inhibition. Circ_CDK14 could sponge miR-520a-3p. miR-520a-3p targeted GAB1 to repress OS cell proliferation and metastasis. Circ_CDK14 knockdown blocked OS tumor growth in vivo. Circ_CDK14 might positively affect OS development by modulating the miR-520a-3p/GAB1 axis.