Making a false oath in the name of Allah is a major sin. The person who deliberately makes a false oath should repent and ask Allah’s forgiveness.
Sheikh HasaninMakhluf, a former grand mufti of Egypt (may Allah bless his soul) stated the following: This oath taken by this person comes under the false oath, which is a major sin because he violated the sanctity of Allah’s name. It was reported by `Abdullah ibn `Amribn Al-`Aas that the Prophet (peace and blessings be upon him) said, “The biggest sins are to associate others in worship with Allah; to be undutiful to one’s parents; and to take a false oath (al-ghamus) “(Al-Bukhari).
The meaning is that those sins are among the major sins but not all the major sins. Actually, the degree of sin varies from one major sin to another, and vice resulted from this oath increases when the person uses it to take property of another wrongfully or to deprive one of a right. It is called ghamus because it engulfs the person in sin, then in Hell. There is no expiation for such an oath, so one must repent sincerely so that the sin may be forgiven. There is no effect given to the above justification to account for lying here because to deliberately lie under an oath is forbidden in any circumstances.