A Study of BOS-580 in Obese Subjects at Risk for, or With Biopsy-confirmed, Nonalcoholic Steatohepatitis (NASH) With an Extension

Inclusion Criteria (Part A and Part B): * Participant is either male or female and 18 to 75 years of age inclusive, at the time of signing the informed consent * Obese participants with body mass index (BMI) of ≥ 27 kg/m\^2 * Hepatic fat fraction (HFF) measured by magnetic resonance imaging derived proton density fat fraction (MRI-PDFF) ≥8% * Liver fibrosis assessment based on a vibration controlled transient elastography (VCTE) liver stiffness measurement (LSM) score of 7.0 to 9.9 kPa (Part A only) inclusive or 7.0 to 20.0 kPa (Part B only) inclusive and Liver injury assessment measured by aspartate aminotransferase (AST) \>25U/L. A qualifying historical biopsy (confirmed eligibility based on the central pathology read) supersedes the LSM, controlled attenuation parameter (CAP) score criteria and AST criteria. * Histopathologically confirmed F2 or F3 stage NASH on a diagnostic liver biopsy performed during Screening or within 6 months prior to the first day of dosing for historical biopsies (Part B only). * History or presence of at least 2 of 4 components of metabolic syndrome: obesity/overweight, dyslipidemia (high triglycerides and/or low high density lipoprotein \[HDL\]), type 2 diabetes with elevated glycated hemoglobin (HbA1c), and hypertension. Inclusion Criteria (Part C): * Participant must have completed the Part B of the study. * Participant willing to undergo liver biopsy at Week 56 * NASH F stage \<F4 at 24 week assessment in Part B Inclusion Criteria (Part D): * BMI of ≥ 25 kg/m\^2 * Liver fibrosis based on assessments taken during screening visit * Participant should be willing and able to undergo liver biopsy during Screening (if a historical biopsy within 12 months prior to Screening is not available) and per protocol as judged by the Investigator. * Other inclusion criteria may apply Exclusion Criteria (Part A and Part B): * Documented clinical, laboratory or radiologic evidence of cirrhosis (compensated or decompensated) * Triglycerides ≥ 500 mg/dL * Change in body weight (more than 5% self-reported OR 5 kg self-reported change during the previous 3 months from Screening, whichever is smaller) * History of type 1 diabetes, diabetic ketoacidosis, or positive glutamic acid decarboxylase (GAD) auto-antibodies (latent autoimmune diabetes in adults) * Hemoglobin A1c \> 9.5% * Participants with a condition that requires substantial anticoagulant medication may not be eligible for the study enrollment (e.g., deep vein thrombosis). Exclusion Criteria (Part C): • Participants that received their 24 week dose in Part B \> 10 weeks prior to enrollment into Part C Exclusion Criteria (Part D): * Other causes of chronic liver disease * Documented evidence or history of decompensated liver cirrhosis. * History of type 1 diabetes or poorly controlled type 2 diabetes. * History of malignancy. * Use of other investigational drugs. * Other exclusion criteria may apply