I read on this site that: "A born-Jewish woman who has had premarital relations may marry a kohen only if all of her partners were Jewish.". How is premarital relations described in Judaism for this purpose? Is it referring to actually having sex? Does oral sex have anything to do with it?
כל הנבעלת לאדם שעושה אותה זונה--בין באונס בין ברצון, בין בזדון בין בשגגה, בין כדרכה בין שלא כדרכה--משהערה* בה, נפסלה משום זונה: ובלבד שתהיה בת שלוש שנים ויום אחד, ויהיה הבועל בן תשע שנים ויום אחד ומעלה.
Whenever a woman engages in relations that cause her to be deemed a zonah, she becomes disqualified as soon as the man's organ enters her* whether she engages in relations against her will or willingly, whether in conscious violation or inadvertently, whether through vaginal or anal intercourse. [This applies] provided she is at least three years old and the man with whom she engages in relations is nine years old or more. (translation from chabad.org)
The Shulchan Aruch uses almost the exact same wording in Even haEzer 6:9.
Please ensure that an actual qualified Rabbi is consulted and given all relevant details when seeking a ruling for a real life question.
* See Issurei Biah 1:10 et seq.