Study to Evaluate the Efficacy and Safety of Oxabact (OC5) in Primary Hyperoxaluria Patients Who Are on Dialysis

Inclusion Criteria: 1. Signed informed consent (as applicable for the age of the subject). An appendix to the informed consent will be signed by patients continuing on treatment after week 14. 2. Male or female subjects ≥ 2 years of age. Subjects have to be able to swallow size 4 capsules twice daily for 6 weeks, or use a gastric tube that allows for administration of size 4 capsules. 3. A diagnosis of PH (as determined by standard diagnostic methods). 4. Patient should be on a stable dialysis regimen for at least two weeks before baseline. 5. Pre-dialysis plasma oxalate ≥40 micromole/L. 6. Patients receiving vitamin B6 must be receiving a stable dose for at least 3 months prior to screening and must remain on the stable dose during the study. Patients not receiving vitamin B6 at study entry must be willing to refrain from initiating vitamin B6 during study participation. Exclusion Criteria: 7. Inability to swallow size 4 capsules twice daily for 6 weeks or using a gastric tube not suited for administration of size 4 capsules via the tube. 8. Ongoing treatment with immunosuppressive medication. 9. The existence of secondary hyperoxaluria, e.g. hyperoxaluria due to bariatric surgery or chronic gastrointestinal diseases such as cystic fibrosis, chronic inflammatory bowel disease and short-bowel syndrome. 10. Use of antibiotics to which O. formigenes is sensitive, including current antibiotic use, or antibiotics use within 14 days of initiating study medication. 11. Current treatment with a separate ascorbic acid preparation. Standard of care vitamin supplement for patients on dialysis is allowed. 12. Pregnancy. 13. Women of childbearing potential who are not using adequate contraceptive precautions. Sexually active females, unless surgically sterile or at least 2 years post-menopausal, must be using a highly effective contraception (including oral, transdermal, injectable, or implanted contraceptives, IUD, abstinence, use of a condom by the sexual partner or sterile sexual partner) for 30 days prior to the first dose of OC5 and must agree to continue using such precautions during the clinical study. 14. Presence of a medical condition that the Investigator considers likely to make the subject susceptible to adverse effect of study treatment or unable to follow study procedures. 15. Participation in any study of an investigational product, biologic, device, or other agent within 30 days prior to the first dose of OC5 or not willing to forego other forms of investigational treatment during this study.