OMRI refers to the Organic Materials Review Instiutute (www.omri.org). It is a non-profit organization in the U.S. that publishes lists of generic materials (e.g., Diatomaecoues Earth) that comply with the standards of the U.S. National Organic Program.
OMRI also lists brand names of specific products have been reviewed by OMRI and found consistent with the standards of the U.S. National Organic Program. These products are submitted to OMRI by manufacturers. It is a purely voluntary process but it's probably fair to say that most reputable manufacturers of organic pest control products in the US seek OMRI approval of their products. Many Canadian companies who market products for the organic industry in the U.S. submit their products to OMRI for approval.