This study was conducted back in 2008 with the use of sesame oil, to be exact, and 10 minutes of gargling daily for two weeks. So what was the conclusion? Oil pulling prior to brushing was effective in reducing bacteria related to tooth decay (specifically the Streptococcus mutans that was found in plaque). Thus, oil pulling can be used "as a preventative home therapy to maintain oral health" .
EFFECT OF OIL PULLING ON S. MUTANS COUNT IN PLAQUE ... (2008)
A REVIEW OF OIL PULLING AND OTHER HOLISTIC APPROACHES TO ORAL HEALTH (2011)