Praise be to Allah, and may peace and blessings be upon the Messenger of Allah. To proceed:
A person named “Abd al-Rahman” should not be addressed by saying “O Rahman,” because “al-Rahman” is a Name exclusively belonging to Allah, unlike shared names that may be used for both Allah and His creatures. The mere utterance of this expression without intending the divine meaning does not necessarily constitute disbelief, but it is preferable to avoid such an address in order to prevent confusion.
The Names of Allah, in terms of whether they are exclusive to Him, can be divided into two categories:
The first category: Names exclusive to Allah, which are not given to human beings as personal names, such as: Allah, the Lord, al-Rahman, al-Ahad, and al-Samad, and similar names.
There is a severe warning in the Sunnah against taking a name that is exclusive to Allah. In the two Sahihs, Abu Hurayrah, may Allah be pleased with him, reported that the Prophet ﷺ said:
“The most contemptible name in the sight of Allah on the Day of Resurrection is that of a man who calls himself King of Kings.”
Abu Ubayd, may Allah have mercy on him, explained that this includes a person taking some of the Names of Allah that are exclusive to Him, such as al-Rahman, al-Jabbar, and al-Aziz.
Ibn al-Qayyim, may Allah have mercy on him, explained the reason for the severity of this warning by stating that Allah dislikes a person imitating Him in a name that belongs exclusively to Him.
Ibn Hajar, may Allah have mercy on him, stated in his commentary on the hadith that the warning concerning “King of Kings” also applies to whatever has the same meaning, including the Names of Allah that are exclusive to Him, such as al-Rahman, al-Quddus, and al-Jabbar.
Accordingly, it is not appropriate to use “al-Rahman” as a personal name for a human being in an unrestricted manner.
Based on the above, a person named “Abd al-Rahman” should not be addressed by saying “O Rahman,” because this expression is a Name exclusive to Allah and may be understood as addressing the person by a Divine Name.
This differs from the use of certain names that are shared between the Creator and the created; “al-Rahman” is among the Names with which Allah is exclusively described, and therefore it should not be treated like names that may properly be used for human beings.
There are names that are not exclusive to Allah and may be used for people, such as Sami‘, Basir, Ali, Hakim, and Rashid, although their meanings and degrees of perfection differ according to whom they are attributed.
Thus, if someone is named Abd al-Karim, it is permissible to address him as “Karim” when the intention is the person himself, because “Karim” is an expression used for both Allah and His creatures.
Likewise, someone named Abd al-Aziz may be addressed as “Aziz,” and someone named Abd al-Hakim may be addressed as “Hakim,” provided that the intended person is the individual and not Allah.
Similarly, “Rahim” is a shared name. Allah says concerning the Prophet ﷺ:
“To the believers he is Most Kind and Most Merciful.” [Qur’an 9:128]
Therefore, a person named “Abd al-Rahim” may be addressed as “Rahim” when the person himself is intended.
As for “al-Rahman,” it does not fall into this category, because it is among the Names exclusive to Allah.
Although it is permissible to address people with compound names by using part of their names when that part is a shared name, it is preferable in the case of “Abd al-Rahman” not to shorten it to “Rahman,” because this may cause ambiguity or confusion.
If people wish to shorten the name or use an affectionate form, they may shorten “Abd” instead, saying, for example, “Ubayd” or “Aboud,” according to customary usage.
A person named “Abd al-Rahman” should not be addressed by saying “O Rahman,” because “al-Rahman” is one of the Names of Allah that is exclusive to Him. As for addressing someone named “Abd al-Karim” as “Karim,” or “Abd al-Aziz” as “Aziz,” or “Abd al-Rahim” as “Rahim,” this is permissible when the person himself is intended, because these expressions are shared names used for both the Creator and the created, while their meanings differ according to what is appropriate for each.
And Allah knows best.