Swearing for the sake of emphasis

Not only it is permitted but it is mustahab to swear in order to emphasize on a true fact, or to prove some fact, or to show its importance. For example the Holy Prophet (s.a.w.s.) speaks after taking the oath: 
"By Allah! Allah does not delay in forgiving, Though you may be lazy in seeking it." 
(Masaalik)

Another example of an oath is the following saying of Amirul Momineen (a.s.): 
"By Allah! If people knew what I know then very few would have laughed and many more would have wept." 
(Masaalik)

There are numerous Quranic verses and the traditions of Masoomeen (a.s.) where oath is taken. All of them are of this same type. They are for emphasis and for proving particular facts.

A person wrote a letter to Imam Muhammad Baqir (a.s.) and asked about something which was being wrongly attributed to him (Imam a.s).

Imam Baqir (a.s.) wrote the following reply, 
"By Allah! What is being attributed is not correct. But under no circumstance do I like to say "By Allah" to disprove it. Yet I regret that such a thing is being said when it really isn't so." 
(Mustadrakul Wasael)

Swearing is Makrooh

We have already described the wajib and Mustahab oaths. Apart from these, in all other situations, it is Makrooh (detestable) to swear. It is irrelevant whether the oath is for something past, present or future. In ordinary situations, for ordinary matters, taking an oath is Makrooh. Swearing for a false thing is certainly Haraam. It is the command of Imam Ja'far as-Sadiq (a.s.):
"Do not swear by the name of Allah, whether you are speaking the truth or lying." 
(Furu Al Kafi)

"Because Allah says in the Holy Quran,
"And make not Allah because of your swearing (by Him) an obstacle to your doing good...""
(Surah Baqarah 2:224)

Hazrat Imam Ja'far as-Sadiq (a.s.) also says, 
"One who swears falsely by Allah for a lie has disbelieved, and one who swear by Allah for truth has sinned, because Allah says: "Do not use Allah for (taking of) your oaths." 
(Furu Al Kafi)

The above quoted traditions could be simply explained thus: 
To swear falsely in the name of Allah is definitely a greater sin and one who indulges in a greater sin, falls down from the highest position of Faith. Due to this some amount of disbelief comes to his heart. Imam (a.s.) has also called a true oath a sin and he has used the word "Ithm" (sin). Since it is prominent among the Mujtahids that a true oath is Makrooh the word "Ithm" would imply something which is very very undesirable.

A Lesson from Hazrat Isa (a.s.)

The book Furu Al Kafi also contains the following report from Imam Ja'far as-Sadiq (a.s.):
"The apostles of Hazrat Isa (a.s.) came to him and said, "O the teacher of good, give us some advice."

He (a.s.) said, "Certainly, Musa, the Prophet of Allah commanded you not to swear falsely by Allah. And I order you not to swear falsely and (nor) truly." 
(Furu Al Kafi)

Oath of respectable objects and personalities

Wherever it is permitted to swear by Allah, it is also permitted to swear by other honorable personalities or respectable objects. In such situations it is allowed to take an oath by the Quran, the Ka'ba, the Prophet (s.a.w.s.) or the Imam (a.s.). In the same way oath upon any other respectable object is also permissible. For example a person may swear by his father or son etc.

Those traditions that prohibit swearing by Allah's name prohibit it for proving ones right. They do not prohibit the proving of a true fact by oath. However when a situation arises where it becomes obligatory to take an oath, then one must swear only by Allah. Oath by any other object or personality will not be absolutely correct and from the Shariat point of view the matter will not end conclusively. Similarly if one has to take an oath for performing a particular action in the future, in this case too, one must swear only by Allah. Any other vow does not have any legal standing.

The oath that is haraam under all circumstances

The oath which is Haraam under all circmstances and which one can never take is that of dissociating with Allah and His religion. For example a man says;
"If I do not perform this particular action, I shall be dissociated with Allah and His religion." Such an oath is certainly Haraam.

In the same way if one says: 
"If I do not do this, I would have disbelieved in the Holy Prophet (s.a.w.s.), or I would have rejected the Mastership of Ali (a.s.), or I would become a disbeliever." Such a vow is also Haraam. It is Haraam whether one wishes to prove the truth or to lay emphasis upon a fact.

The Holy Prophet (s.a.w.s.) heard a person taking such an oath. He (s.a.w.s.) said, "Woe be unto you, if you leave the religion of Muhammad (s.a.w.s.) then which religion would you follow?"

The narrator says that the Holy Prophet (s.a.w.s.) did not speak to this man till the end of his life. 
(Al Kafi)

Imam Ja'far as-Sadiq (a.s.) informed Yunus Ibne Zabyan: 
"O Yunus! Do not speak about dissociating from us in an oath. One who takes oath from it, whether for a true thing or a falsehood, he really becomes dissociated from us." 
(Al Kafi)