Background Oral squamous cell carcinoma (OSCC) frequently invades the jaw. The exact mechanism of bone invasion remains unclear. This study investigates the role of osteoclasts and RANKL/OPG/RANK in the development of bone invasion in OSCC. Methods OSCC-patients treated with resection were included and divided in three groups; Non-invasion (NI-group), erosion (E-group) and bone invasion (I-group). Tissue-sections were stained with Cathepsin K (for counting osteoclasts), RANKL, OPG and RANK. Staining intensity was scored in tumor-front, tumor-center, tumor-backside and normal mucosa. Immunohistochemistry and qPCR for RANKL/OPG/RANK was performed in five head-and-neck SCC organoids to correlate protein and mRNA-expression levels. Results The mean number of osteoclasts in Cathepsin K stained sections in the NI-group was 3.09 (1.12-5.05; 95%CI), in the E-group 6.15 (4.04–8.25; 95%CI) and in the I-group 10.58 (5.81–15.34; 95%CI). Compared to normal mucosa, the expression in all tumor regions was higher for RANKL, in most tumor regions for OPG and not higher for RANK. RANKL-expression in tumor-front was higher than expression in tumor-backside (I-group). RANK-expression in the tumor-front and the tumor-center was higher than expression in tumor-backside in all groups. qPCR showed a 20-43x higher RANKL-mRNA expression in 3/5 tumor organoid samples compared to a normal squamous cell organoid line and no higher mRNA-expression of OPG and RANK. There was no correlation between protein and mRNA expression in the HNSCC organoids. Conclusion The number of osteoclasts and their regulating proteins RANKL/OPG/RANK differ between OSCC patients with and without bone invasion. Bone invasive OSCCs have more osteoclasts and express more RANKL in tumor-front, which suggest that OSCC’s induce bone invasion by stimulating osteoclast activation by regulating the production of RANKL/OPG/RANK proteins.