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The effects of carboxymethylation (CM), hydroxypropylation (HP), and cross-linking (CL) on the physicochemical, thermal, and microstructural properties of oat dietary fiber (ODF) were investigated. ODF extracted from defatted oat flour had 66.14% total dietary fiber, 3.42% protein, 5.15% ash, and 1.86% starch. Chemical modifications (CM, HP, CL) significantly improved the SDF but decreased the IDF content of ODF. Results displayed that all the chemical treatments significantly (p < 0.05) improved the swelling capacity, oil absorption capacity, and water absorption capacity of ODF with carboxymethylation being the highly influential. The CM significantly increased the solubility (7.15%), while CL (4.78%) and HP (4.17%) significantly (p < 0.05) decreased the solubility of ODF. The color evaluation showed a decrease in the L* value of ODF after all three modifications, resulting in an increase in the brown color of ODF. XRD results demonstrated an increase in the crystallinity value of ODF after these modifications (19.23–28.94%). DSC results reflected an increase in decomposition/peak temperature of cross-linked ODF showing enhanced thermal stability of such type of modified DF. The changes in physicochemical, microstructural, and thermal properties of ODF after chemical modifications could be attributed to the variations in chemical composition and structure as demonstrated by XRD, FTIR, and SEM analysis. [Display omitted] • Carboxymethylation greatly affected the physicochemical properties of oat fiber. • Cross-linked oat dietary fiber showed improved thermal stability. • Crystallinity of modified oat dietary fiber increased significantly. • Highest SDF content was found in carboxymethylated dietary fiber. [ABSTRACT FROM AUTHOR]