Lung Cancer: The most common cause of SVCO, particularly in adults, due to the tumor's proximity to the superior vena cava and its potential to compress or invade the vein.

Lymphoma: Both Hodgkin's and non-Hodgkin's lymphoma can cause SVCO by compressing the superior vena cava with enlarged lymph nodes in the mediastinum.
Thrombosis: Central venous catheters, pacemakers, or other intravascular devices can lead to thrombosis, which may obstruct the superior vena cava.
Metastatic Cancer: Cancers that metastasize to the mediastinum, such as breast cancer, can also cause SVCO by compressing the superior vena cava.

Pulmonary Embolism: Although less common, a large pulmonary embolism can cause SVCO by obstructing blood flow through the superior vena cava or by causing a significant increase in pressure within the pulmonary vasculature.
Aortic Dissection: An aortic dissection involving the ascending aorta can compress the superior vena cava, leading to obstruction.
Mediastinal Fibrosis: A rare condition characterized by the deposition of collagen and fibrous tissue in the mediastinum, which can compress vital structures including the superior vena cava.

Thyroid Goiter: A large thyroid goiter can extend into the thoracic cavity and compress the superior vena cava.
Granulomatosis with Polyangiitis (formerly known as Wegener's Granulomatosis): A systemic vasculitis that can cause inflammation and compression of the superior vena cava.
Sarcoidosis: A condition that can cause granulomatous inflammation in various parts of the body, including the mediastinum, potentially leading to SVCO.
Congenital Anomalies: Rare congenital anomalies, such as a double superior vena cava or other vascular malformations, can predispose to SVCO.