Active middle ear implants such as the Vibrant Soundbridge (VSB) represent effective treatment options for patients with conductive or mixed hearing loss when conventional hearing aids are insufficient. Nevertheless, postoperative speech intelligibility varies considerably, even among patients with comparable audiometric profiles. This first part of a two-part prospective observational study aimed to analyze outcomes and identify clinically accessible predictors of postoperative speech intelligibility. Twenty adults who underwent unilateral VSB implantation were included. Audiological assessments comprised postoperative bone conduction (BC), Vibrogram (VIB), free-field thresholds, maximum unaided speech intelligibility (WRS max), and aided speech intelligibility at 65 dB SPL (WRS 65dB). Additional parameters included coupling efficiency, effective gain, dynamic range, loudness scaling, and patient-reported outcomes (PROMs; IOI-HA, SSQ12). Key predictors of WRS 65dB were identified using Spearman correlation analysis (ρ). The mean postoperative WRS 65dB was 81.5% (SD = 9.0%). The strongest correlations were observed with age (ρ = - 0.63) and two surrogate parameters of cochlear reserve - WRS max (ρ = 0.56) and postoperative BC PTA4 (ρ = - 0.52). PROMs confirmed high subjective benefit and displayed correlations with audiological outcomes. Age, WRS max and postoperative BC PTA4 will serve as the basis for the AI-based prediction model presented in part II. The correlation of PROMs with audiological outcomes highlights the importance of integrating objective and subjective measures in hearing rehabilitation.