Oncocytic adenoma (Hürthle cell adenoma—HCA) is a rare benign thyroid tumor characterized by follicular architecture with more than 75% tumor cells configured as oncocytes (Hurthle cells), which demonstrate intracytoplasmic accumulation of numerous dysfunctional mitochondria. Designated as HCA, oncocytic adenoma, oncocytic cell adenoma, Askanazy cell adenoma, eosinophilic adenoma, or oxyphilic cell adenoma, the exogenous factors influencing the emergence of this neoplasm remain undefined. Although HCA is a benign tumor, it has, similar to all adenomas, the potential to become malignant, transforming to the malignant counterpart, oncocytic carcinoma of the thyroid (Hurthle cell carcinoma—HCC), and to metastasize to other bodily organs. Thus, this tumor should be excised promptly to prevent the disease from progressing further. A total thyroidectomy is often performed, but some patients may only undergo hemithyroidectomy or lobectomy without the need for any adjuvant therapy due to its good prognosis. This chapter discusses the risk factors and clinical significance of oncocytic adenoma, workup of the tumor including imaging techniques (ultrasonography and compute tomography scan) and cyto-histopathological examination, in addition to the differential diagnosis of this neoplasm. Moreover, this chapter explores the association of autoimmune thyroiditis especially chronic lymphocytic thyroiditis (Hashimoto’s thyroiditis) with oncocytic/HCA and its impact on management and prognosis.