Anal sex , Sex during menstruation and nifas, Sex that would be harmful or prolong recovery from an illness

Oral Sex Is oral sex permitted?
Books of fiqh tend to be quite dry when it comes to bedroom behavior. What you usually find is in the form "everything is permissible, except for x, y, and z" instead of a detailed checklist of things to try if you wish. While the traditional approach of generality is fine as a teaching tool for students of knowledge who would be asked specific questions, it is less effective when laymen have no one to ask, when the only people available to ask mean well and know a bit more than the questioner but are laymen themselves, or when laymen go to the books directly.
So, since the issue of oral sex is frequently asked it seems necessary to give a general answer. I apologize that the questioner is going to get less and more than he originally asked for.

The basic list of things that are unlawful between husband and wife include:

Anal sex

Sex during menstruation and nifas

Sex that would be harmful or prolong recovery from an illness

When books talks about what is lawful, they typically mention that a husband and wife may pleasure one another in any way they wish other than the above mentioned things. Although not specific to sex, we can add the following:

Swallowing filth

Needlessly getting filthy These things are obvious.

Regarding the issue of oral sex, if it can be done in such a way that no filth is involved, then from the perspective of lawful and prohibited it in and of itself would be considered lawful. But the term "oral sex" covers a wide range of activities, and there are differences between the various schools of fiqh regarding the purity and filth of bodily fluids, namely: sperm and vaginal secretions; there seems to be agreement among the Hanafis, Shafi`is, and Hanbalis that pre-ejaculatory fluid is filthy. Something that entirely escapes the problem of filth is a man kissing the outer parts of his wife's genitals and sucking her clitoris. The first is mentioned in many books of Hanbali fiqh, saying that it is permissible before penetration and offensive after; and to clarify: the issue mentioned is not restricted to the outer parts. The second is mentioned in the Shafi`i work Fath Al-Mu`in and its meta-commentary `Iyanat Al-Talibin. For further reference, look through the index of the book of marriage for the sections concerning looking at members of the opposite sex and the sections conerning bedroom behavior .
Based on the basic guidelines above and based on the example of a man kissing his wife's genitals, it should also be lawful for a wife to kiss her husband's penis as long as she avoids any areas where there is pre-ejaculatory fluid.
But what Westerners usually have in mind when they talk about oral sex are fellatio and cunnilingus. Regarding pre-ejaculatory fluid, the only opinion I have ever seen is that it is haram. Additionally, even though the Shafi`is and Hanbali schools both consider sperm to be pure, they both agree that it is unlawful to swallow. This makes fellatio very highly likely to be unlawful. I suppose an enthusiastic couple could always use a condom.
As for cunnilingus: there are differences of opinion regarding whether or not vaginal secretions are pure or filthy, before and after penetration, and depending on where they are secreted from. Because of this, it is best avoided.
Should a couple decide to perform these acts, they must be sure to rinse out their mouths afterwards. This is something they should be doing in any case, and it is an essential part of purification for Hanbalis.
For added benefit and so we don't need to repeat this topic again: Hanbali books mention that it is lawful for a husband to achieve ejaculation by his wife masturbating him, and by rubbing his penis between her thighs or breasts. They also mention that while anal sex is absolutely unlawful, it is lawful to place a finger on the anus as long as it does not penetrate. One of the reasons for mentioning these detailed examples is to show that they were not unknown to the scholars, and in sha Allah to add strength to the idea that the general rulings mentioned in the books are sufficient for working out these detailed cases.
Up to here the topic concerns the lawful and the prohibited. But this issue is a completely separate from whether these acts should be done, and whether one spouse can compel the other one to do them.
When I asked my sheikh about these very issues he pointed out that while each spouse is responsible to provide for the other spouse's sexual needs and to keep them chaste, it is absolutely unlawful to abuse one another, mentally or physically. He also points out that Allah Most High in the Qur'an orders husband and wife to act amicably and well towards one another, and one needlessly forcing the other to do something--even when they have the right to demand it--goes against this principle. I once asked whether or not it was disobedience [nushuz] if husband demanded fellatio from his and she refused. His answer was that she is ordered to submit herself to him for intercourse: something she has not denied him here.
Even if people find this whole topic disgusting in practice, we should take note that the example given in the books is one where it is the wife who receives physical pleasure, not the husband, and that the location mentioned is the one most likely to bring about an orgasm. So much for the idea that Islam totally ignores the sexual needs of women, rendering them as little more than house keepers and sex toys for the husband's every whim. That the example of oral sex in fiqh books is for her pleasure and satisfaction should not go unnoticed.
And Allah knows best.