In this study, a solvothermal method was used to synthesize a composite of iron oxide nanostructures on carbon nanotubes (CNTs), which was applied as a resistive sensor for hydrogen gas (H2) detection. The nanocomposite was produced with three different iron oxide concentrations (Fe1@CNT, Fe2@CNT, and Fe3@CNT) to investigate the effect of iron species on CNTs and their interaction with hydrogen. Electron microscopy revealed that increasing iron oxide content led to the deterioration of the CNT walls. Raman and FTIR spectra confirmed the predominant presence of α-Fe2O3 (hematite) on the CNTs, while XPS analysis verified the presence of multiple iron oxides species. High-resolution XPS of the Fe 2p region indicated the existence of Fe3O4 (magnetite), Fe2O3 (hematite), and FeO (iron(II) oxide) associated with the CNTs. The sample with the lowest iron oxide concentration (Fe1@CNT) showed a 45% sensor response to hydrogen in a dry air atmosphere and the longest recovery time, suggesting a stronger interaction between hydrogen and the nanocomposite. Molecular dynamics simulations and density functional theory calculations further revealed that the presence of iron oxide on the CNT surface significantly altered its electronic properties, particularly by introducing more electronic states near the Fermi level, which enhanced electronic exchange between H2 and the carbon nanotube containing iron oxide.