The highest risk associated with the use of oral contraceptives is in:
Hepatic necrosis
Permanent sterility
Thromboembolic disorders
Cancer of breast
Serious Complications

Leg vein thrombosis and pulmonary embolism: The older preparations increased the incidence of venous thromboembolism, but this is found to be less marked (only 2-3 patients per 1000 women years) with the newer reduced steroid content pills. However, even low-dose pills pose significant risk in women > 35 years of age, diabetics, hypertensives and in those who smoke. The excess risk is due to the estrogen component of the OC, and normalizes shortly after stopping the OC.
Rise in BP: Occurred in 5-10% women taking the earlier pills. The again is less frequent and smaller in magnitude with the low-dose pills of today. If the BP rises, best is to stop OCs: the BP normalizes in the next 3-6 months. Both the estrogen and progestin components are responsible for this effect, probably by increasing plasma angiotensinogen level and renin activity which have been found to be raised during OC use. Aldosterone secretion is enhanced resulting in salt and water retention.

Reference: Essentials of Medical Pharmacology Eighth Edition KD TRIPATHI page no 351