Background/Objectives: Metabolic dysfunction-associated steatotic liver disease (MASLD) is closely associated with obesity and insulin resistance (IR). Identifying characteristics that predict a higher risk of fibrosis using noninvasive methods is particularly important. Methods: We performed a secondary analysis of data from an RCT of 48 patients after one anastomosis gastric bypass (OAGB) surgery, supplemented with specifically formulated probiotics and micronutrients or control treatment for 12 weeks. Patients were categorized using alanine aminotransferase (ALAT; >35 U/L for women, >50 U/L for men), higher NAFLD fibrosis score (NFS) > −1.455), and IR (HOMA-IR > 2.0). This trial was registered at Clinicaltrials.gov (ID: NCT03585413). Results: Abnormal ALAT was associated with high triglycerides, blood pressure (BP), glucose, and fatty liver index (FLI). NFS > −1.455 was linked to higher age, body mass, waist circumference, and FLI, and lower albumin and platelet count. HOMA-IR > 2.0 was associated with higher BP and triglycerides, lower HDL-cholesterol, higher serum transaminases, and higher probabilities of steatosis and fibrosis. Twelve weeks postoperatively, patients with NFS > −1.455 showed greater reductions in body mass, systolic BP, serum insulin, and HbA1c, whereas those with NFS ≤ −1.455 showed improvements in FLI and lipid metabolism but had high glucose concentrations. Patients with HOMA-IR ≤ 2.0 also had high glucose concentrations. Conclusions: The evaluation of common biomarker scores for fibrosis and IR may help clinicians to recognize severe NAFLD and improve the outcomes of OAGB surgery.