If the purport of your question is to inquire whether it is considered zina to engage in oral sex with anyone other than one’s married partner, the answer is most categorically and definitely, yes.
In other words, it is nothing but zina, pure and simple. For according to the explicit teachings of Islam, it is absolutely sinful to establish sexual intimacy or derive sexual satisfaction from anyone other than one’s own lawfully married spouse.
If, on the other hand, you mean to ask whether married couples are allowed to derive pleasure in this way from each other, then the answer is, yes, provided they are doing so consensually. To derive sexual satisfaction from anyone other than one’s marriage partner in any way is considered zina in Islam. This is according to the explicit commandment of Allah: (And come not near unto adultery. Lo! it is an abomination and an evil way.) (Al-Israa' 17:2).
The Prophet (peace and blessings be upon him) further said, “Guard your private parts [that is, keep yourself chaste and pure by abstaining from any sexual relations with anyone] except from your lawful spouses.”(Reported by Abu Dawud and others).
I pray to Allah to grant that we love faith as well as thoughts, words, and actions that enhance it; and make us detest sins of mind, speech, and actions.