Osr1 regulates macrophage-mediated liver inflammation during nonalcoholic steatohepatitis development by modulating cell polarization and metabolisms. Targeting macrophage Osr1 can be a promising treatment strategy for nonalcoholic steatohepatitis. BACKGROUND & AIMS: Liver macrophage-mediated inflam-mation contributes to the pathogenesis of the nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH). Odd skipped-related 1 (Osr1) is a putative transcription factor previously reported to be involved in NASH progression; however, the underlying mechanisms remain unknown. The current study focused on the role of Osr1 in macrophage polarization and metabolism and its associated functions in the inflammation-induced pathogenesis of NASH. METHODS: OSR1/Osr1 expression patterns were compared in normal and NASH patients and mouse livers. NASH was established and compared between hepatocyte-specific Osr1 knockout (Osr1(Delta Hep)), macrophage-specific Osr1 knockout (Osr1(Delta M phi)), and wild-type (Osr1(F)) mice fed with 3 different chronic obesogenic diets and methionine choline-deficient diet. Using genetic and therapeutic strategies in vitro and in vivo, the downstream targets of Osr1 and the associated mechanisms in inflammation-induced NASH were established. RESULTS: Osr1 was expressed in both hepatocytes and mac-rophages and exhibited different expression patterns in NASH. In NAFLD and NASH murine models, deleting Osr1 in myeloid cells (Osr1(Delta Mf)), but not hepatocytes, aggravated steatohepatitis with pronounced liver inflammation. Myeloid Osr1 deletion resulted in a polarization switch toward a pro-inflammatory phenotype associated with reduced oxidative phosphorylation activity. These inflamed Osr1DMf macrophages promoted steatosis and inflammation in hepatocytes via cytokine secretion. We identified 2 downstream transcriptional targets of Osr1, c-Myc, and PPAR gamma and established the Osr1-PPAR gamma cascade in macrophage polarization and liver inflammation by genetic study and rosiglitazone treatment in vivo. We tested a prom-ising intervention strategy targeting Osr1-PPAR gamma by AAV8L-delivered Osr1 expression or rosiglitazone that significantly repressed NAFLD/NASH progression in Osr1(F) and Osr1(Delta M phi) mice. CONCLUSIONS: Myeloid Osr1 mediates liver immune homeo-stasis and disrupting Osr1 aggravates the progression of NAFLD/NASH.