Oil pulling has received little study and there is little evidence to support claims made by the technique's advocates. In one small study, sesame oil was found to be effective at reducing plaque and oral bacterial load, but was less effective than chlorhexidine (an antisepticmouthwash); the health claims of oil pulling have otherwise failed scientific verification or have not been investigated. The National Center for Health Research states that "it's still unclear whether or how the practice actually works to get rid of bad bacteria in our mouths. It's also unknown what the long term effects on oral and overall health may be." 
In traditional Ayurveda, gargling treatments like kavala graha and gandusha are used to treat imbalances of various doshas. Ayurveda does not recommend general treatments blindly for everyone, but rather, health is held to be very individualistic, and, the dominant dosha in both the individual and nature determines health care, including dental health. As per Ayurvedic literature, sesame oil is one among many medicinal fluids recommended for daily preventive use and/or seasonal use to reduce dryness (vata dosha) of the mouth and reduce inflammation and burning sensation in the mouth. In case of specific issues, Ayurvedic practitioners might also suggest other treatments such as coconut oil and sunflower oil or other herbalized oils after proper diagnosis of the specific ailment or dosha.
The phrase "oil pulling" and usage in its current form was popularized in the early 1990s by one of the early adopters, Tummala Koteswara Rao in Bangalore, South India. Rao actively evangelized oil pulling as an ancient Ayurvedic practice.
Traditionally, sesame oil was used for oil pulling, but recent endorsements by celebrities has resulted in increasing popularity of coconut oil based oil pulling in the Western world.
Purported mechanism of action
Even after several (limited) studies, the exact mechanism of action of oil pulling therapy is still not clear.
The suggestion is that oil provides a surface layer that prevents plaque or bacteria adhering to teeth.
It is also suggested that by increasing the secretion of saliva, oil pulling uses the salivary glands in the mouth as a detoxifying organ - the saliva can trap the toxin within the oil particles.
It is also suggested that the prolonged and forceful mechanical action could play a part in dislodging bacteria and undigested particles from the deep crevices within the mouth.
Oil pulling with sesame seed oil moisturizes gums, which can provide a measure of relief to those suffering from dry mouth. Dry mouth is known to increase bacteria growth.
Dentists remain skeptical of the claimed benefits behind oil pulling. Reliable scientific evidence of the benefits and risks is scarce and American Dental Association states that insufficient research has been done on oil pulling. Rather than oil pulling, the ADA recommends brushing the teeth twice a day, flossing, and the use of an antiseptic mouthwash. The Canadian Dental Association responding to published research, has stated that "We sense oil pulling won't do any harm, we're not convinced there are any particular benefits to it."
A 2013, in vitro, study found that oil pulling with olive oil, safflower oil, or linseed oil had no effect on microbial colonization of the enamel. The authors concluded that it could not be recommended for biofilm reduction.
Against the background of current scientific and empirical knowledge, edible oils might be used as oral hygiene supplements but a decisive benefit for the oral health status is questionable  The drawbacks highlighted by medical professionals and experts are:
Oil pulling cannot replace care from a qualified dentist, and any delays in going to the dentist might make it difficult to treat mouth problems.
It consumes more time than conventional alcohol-free, antiseptic mouthwash
The oil has to be spit into the garbage can, not the sink. Coconut oil if it solidifies can clog the pipes.
Ayurvedic experts warn of negative side effects - if improper technique is used - such as dry mouth, excessive thirst, muscular stiffness, exhaustion and loss of sensation or taste in the mouth.
^Kim, Jae Yeol; Jung, Jae Woo; Choi, Jae Chol; Shin, Jong Wook; Park, In Won; Choi, Byoung Whui (1 February 2014). "Recurrent lipoid pneumonia associated with oil pulling [Correspondence]". The International Journal of Tuberculosis and Lung Disease18 (2): 251–252. doi:10.5588/ijtld.13.0852.