The ostium of the Fallopian tube

Fallopian tube

The Fallopian tubes, also known as oviducts, uterine tubes, and salpinges are two very fine tubes lined with ciliated epithelia, leading from the ovaries of female mammals into the uterus, via the utero-tubal junction...

(plural ostia) may refer to the proximal or distal opening of the tube.

The proximal tubal opening (ostium or os) is located within the uterus at the uterotubal junction

Uterotubal junction

The Uterotubal junction or Utero-tubal junction is the connection between the endometrial cavity of the uterus and the fallopian tube at the proximal tubal opening, the beginning of the intramural part of the fallopian tube...

 and accessible via hysteroscopy. Occlusion at this opening is referred to as proximal tubal occlusion.

The distal tubal opening (or abdominal ostium) is the opening in the infundibulum of uterine tube

Infundibulum of uterine tube

The third part of the uterine tube is the infundibulum. It terminates with the ostium of Fallopian tube, surrounded by fimbriae, one of which is attached to the ovary. Together, the infundibulum and fimbria find the oocyte after ovulation.-External links: - "The Female Pelvis: The Oviduct"...

 into the abdominal cavity

Abdominal cavity

The abdominal cavity is the body cavity of the human body that holds the bulk of the viscera. It is located below the thoracic cavity, and above the pelvic cavity. Its dome-shaped roof is the thoracic diaphragm , and its oblique floor is the pelvic inlet...

. In ovulation

Ovulation

Ovulation is the process in a female's menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum . Ovulation also occurs in the estrous cycle of other female mammals, which differs in many fundamental ways from the menstrual cycle...

, the oocyte

Oocyte

An oocyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell which undergoes a mitotic...

 enters the Fallopian tube through this opening.
It is surrounded by fimbriae

Fimbria (female reproductive system)

In the female reproductive system, the fimbria is a fringe of tissue around the ostium of the Fallopian tube, in the direction of the ovary....

, which help in the collection of the oocyte. Occlusion of this opening is referred to as distal tubal occlusion.