Corpus luteum cyst: This is the most likely diagnosis because the patient is asymptomatic and uses oral contraceptives. Corpus luteum cysts are common in women of reproductive age, especially those on oral contraceptives, which can affect ovulation and corpus luteum formation.

Benign cystic teratoma: Although less common than corpus luteum cysts, benign cystic teratomas (dermoid cysts) are a frequent cause of adnexal masses in young women and should be considered.
Endometrioma: While endometriomas are more commonly associated with symptoms like pelvic pain, they can be asymptomatic and are a possible cause of adnexal masses in women of reproductive age.

Dysgerminoma: Although rare, dysgerminoma is a type of ovarian cancer that can present as an adnexal mass. It is crucial to consider malignant causes, even if they are less likely, due to the significant implications of missing a cancer diagnosis.
Mucinous cystadenoma: While typically benign, mucinous cystadenomas can be large and, in rare cases, may have malignant potential. Their consideration is important to rule out more serious conditions.

Other ovarian tumors (e.g., serous cystadenoma, ovarian fibroma): These are less common causes of adnexal masses in young women but should be considered in the differential diagnosis to ensure a comprehensive evaluation.
Tubular or paraovarian cysts: These are less frequent causes of adnexal masses and might not be the first consideration but are part of the broad differential diagnosis for adnexal masses.