This study evaluated the reliability and validity of the Oswestry Disability Index (ODI) among patients undergoing lumbar spinal surgery. A total of 1,515 patients who underwent lumbar spinal surgery between 2018 and 2021 were included. The ODI was administered within 2 months before surgery. The internal consistency of the ODI was assessed using Cronbach's alpha, which was found to be good (0.87). Exploratory and confirmatory factor analyses revealed a unidimensional structure, with item loadings ranging from moderate to substantial. The main factor "disability" showed moderate to substantial correlations with individual items, with the highest correlations for "traveling," "personal care," and "social life," and the lowest for "standing." The study concluded that the ODI is a unidimensional and internally consistent scale that can be used to assess disability severity in patients undergoing lumbar spinal surgery. The most important items for defining disability severity were "traveling," "social life," "sex life," and "personal care," while "walking" and "standing" were the least important. The generalizability of the results may be affected by the heterogeneity and modest size of the studied cohort. The study also found that the ODI is a reliable and valid scale in a heterogeneous population of patients expecting spinal surgery, and it can describe disability severity from two directions – as a composite score for a quick assessment or as a functional profile containing individual item scores for a more thorough evaluation on a personal level.This study evaluated the reliability and validity of the Oswestry Disability Index (ODI) among patients undergoing lumbar spinal surgery. A total of 1,515 patients who underwent lumbar spinal surgery between 2018 and 2021 were included. The ODI was administered within 2 months before surgery. The internal consistency of the ODI was assessed using Cronbach's alpha, which was found to be good (0.87). Exploratory and confirmatory factor analyses revealed a unidimensional structure, with item loadings ranging from moderate to substantial. The main factor "disability" showed moderate to substantial correlations with individual items, with the highest correlations for "traveling," "personal care," and "social life," and the lowest for "standing." The study concluded that the ODI is a unidimensional and internally consistent scale that can be used to assess disability severity in patients undergoing lumbar spinal surgery. The most important items for defining disability severity were "traveling," "social life," "sex life," and "personal care," while "walking" and "standing" were the least important. The generalizability of the results may be affected by the heterogeneity and modest size of the studied cohort. The study also found that the ODI is a reliable and valid scale in a heterogeneous population of patients expecting spinal surgery, and it can describe disability severity from two directions – as a composite score for a quick assessment or as a functional profile containing individual item scores for a more thorough evaluation on a personal level.