Expiation for Oral Sex

There is no doubt that what you have done is a kind of Zina (unlawful sexual activity ), although it is not the worst kind, so you must repent sincerely to Allah , give up this sin, regret what has happened and be determined never to go back to it.

Avoid bad friends and bad places, lower your gaze and keep your distance from women to whom you are not related (non-Mahram). 

If you repent to Allah, He will accept your repentance . You are still a Muslim, but you have committed an act of disobedience towards Allah by doing this sinful act, so come back to your Lord and seek His forgiveness for what you have done. 

Do more good deeds to expiate your bad deeds and cancel them out, do those things that will help you to remain chaste, and hasten to marry according to Islam. Finally, we will leave you with a story from which you may learn a valuable lesson:

Ibn Mas`ud (may Allah be pleased with him) reported that a man came to the Prophet (peace and blessings of Allah be upon him) and told him that he had kissed or touched a woman, or something similar, as if he was asking how he could expiate for this (according to another report: a man had done something with a woman that fell short of actual intercourse. He came to `Umar ibn Al-Khattab, who regarded it as something very serious; then he came to Abu Bakr, who regarded it as something very serious. Then he came to the Prophet (peace and blessings of Allah be upon him)). Then Allah revealed the verse (interpretation of the meaning): And perform prayer at the two ends of the day and in some hours of the night. Verily, the good deeds remove the evil deeds (i.e., small sins). That is a reminder (advice) for the mindful (those who accept advice). [Hood 11:114]. The man asked, Is this concerning me, O Messenger of Allah? He said: It is concerning whoever of my Ummah does this. (Reported by Muslim, may Allah have mercy on him, in his Sahih, 4963)

According to a report narrated by Abdullah (may Allah be pleased with him), a man came to the Prophet (peace and blessings of Allah be upon him) and said, O Messenger of Allah, I fondled a woman in the furthest outskirts of Madinah but I did not actually have intercourse with her. Here I am, judge me as you wish. Umar said to him: Allah had covered you, you should have covered yourself (i.e., you should not have spoken of it). The Prophet (peace and blessings of Allah be upon him) did not reply at all, so the man got up and left. The Prophet (peace and blessings of Allah be upon him) sent a man to follow him, call him and recite to him the verse (interpretation of the meaning): And perform prayer at the two ends of the day and in some hours of the night. Verily, the good deeds remove the evil deeds (i.e., small sins). That is a reminder (advice) for the mindful (those who accept advice). [Hud 11:114]. A man who was present said: O Prophet of Allah, is this just for him? He said, No, it is for all the people. (Reported by Muslim, 4964)

And Allah knows best.