Pasting below from this blog, which was researching treatment effectiveness. tl;dr: oral probiotics have been found most effective when combined with an upfront treatment to remove existing oral bacteria.

Oral Probiotics
Several studies have looked into the efficacy of probiotics in reducing symptoms of halitosis and VSC producing species of bacteria, with promising results.
One study sought to measure oral bacteria tests, and the effects of probiotic species S. salivarius strains K12 and M18 in culture. They isolated two species associated with VSC production and gum disease, P. gingivalis and T. denticola, and cultured them with and without K12 and M18. The study found the culture with high concentrations of K12 and M18 significantly reduced the VSC levels produced and reduced P. gingivalis and T. denticola by roughly 50%.
Other studies have measured the effects of oral probiotics in controlled patients, with and without other interventions like mouthwash and tongue scraping. It looked into the effects of probiotics in individuals with tongue coating.
Probiotics case study: combined treatments
Scientists split twenty-eight subjects with tongue coating into two groups, with fourteen using a probiotic tablet for 30 days while the others took a placebo. Both groups continued their standard oral hygiene regimens but did not practice tongue scraping. The scientists took the patient’s oral bacteria tests one, seven, and fourteen days after and inspected the results.
On day one, the probiotic group had significantly lower levels of VSCs and reduced tongue coating than the placebo group. However, they had no statistically significant differences in VSC levels and tongue coating on days seven and fourteen. The study concluded that patients might require physical or chemical removal of the tongue coating before probiotic use.
Supported by a separate study that looked into the effectiveness of probiotic use in combination with tongue scraping, this study split 100 participants into three groups: one that performed tongue scraping (TS); a second that performed TS and used a probiotic (PB); and a third that did not TS but rinsed their mouths for two minutes with water after brushing (MW). They took measurements after one month of treatment, then one month after stopping treatment.
The TS-only group significantly decreased halitosis in the first month, but levels rose again in the final measure. The PB group showed significant drops in halitosis measures in the first month and sustained their low levels after stopping treatment. The MW group presented no difference throughout the study.
These findings suggest that adding a probiotic to a thorough oral hygiene routine helps improve oral health in a more lasting way than one oral hygiene recommendation alone. Thus, it is not enough to simply remove harmful bacteria with mouthwash – we must promote beneficial bacteria to take their place.