Oregano is a herb have been used in folk medicine since ancient times, including by Hippocrates.
Survey results have found positive benefits for readers of this blog (71% improved).
- “the most susceptible strains to Origanum vulgare L. essential oil were: E. coli 1 , E. coli 2 , S. aureus 3 and C. albicans1-3 . Inhibition of C. albicans filamentation was detected at 0.45 μl ml-1″ 
- Antimicrobial activity of Basil, Oregano and Thyme essential oils.
- “The oregano oil antimicrobial effect is accredited to carvacrol and thymol, while its antimicrobial spectrum is broad including bacteria (methicillin-resistant S.aureus, Listeria innocua, L. monocytogenes, A.baumannii, K. pneumoniae, Citrobacter freundii, S. enteritidis, S. typhimurium, E. coli, E. coli O157:H7, P. vulgaris, P. aeruginosa, P. fluorescens, Y. enterocolitica, Bacillus subtilis, B. cereus, Serratia liquefaciens, Lactobacillus carvatus, Lactobacillus sakes), fungi (Aspergillus spp. Candida spp.) and parasites (Blastocystishominis, Entamoeba hartmanni, Endolimax nana).”
- “In the ileum, the Oregano group did not exhibit the sharp decline (p < 0.001) in the lactic acid bacteria population observed in the other two experimental groups.” So lactic acid friendly!
- ” No negative effect on growth or metabolic activity of lactic acid bacteria Streptococcus thermophilus CRL 728 and CRL 813, Lactobacillus delbrueckii subsp. bulgaricus CRL 656 and CRL 468, and Lactococcus lactis subsp. lactis CRL 597 up to the maximum concentration (200μg/g) assayed was observed. ” 
No studies could be located for IBS/FM/CFS.
- “Oregano oil exhibited the highest antibacterial activity followed by manuka and thyme oils…The results obtained in this study highlight that exposure to sub-inhibitory concentration of EO modifies strains enterotoxins production and EOs susceptibility profile.” 
- “Results showed that protein, pH, aw, presence of beef extract, sodium lactate and nitrates did not influence their antimicrobial effect. In contrast, the presence of pork fat had a negative effect against both [Garlic and Oregano] Essential Oils associated with their dilution of the lipid content. The addition of food phosphates also exerts a negative effect against EOs ” 
This is a very easy herb to obtain any where in the world. 454 gm of ground organic oregano cost $15 on Amazon. There is no dosage information available, but I suspect 8 “00” capsules a day is fine. Based on what bacteria is effective against and readers’ experience, it is definitely recommended.