Background Ossiculoplasty hearing outcomes are impacted by numerous variables, including ear environment risk factors and modifiable surgical factors. This presentation A) analyses which middle ear factors are most impactful on ossiculoplasty hearing outcomes by presenting a novel statistically validated Ear Environment Risk (EER) grading scale and B) presents data regarding the impact of modifiable surgical factors on ossiculoplasty hearing outcomes. Methods: A multi-institutional database of ossiculoplasty outcomes from 2011-2019 was created from 6 tertiary-care medical centres. Multiple regression statistical analysis was applied to determine ear environment risk factors that are independently associated with postoperative pure-tone average air-bone gap (PTA-ABG). These factors were used to create a statistically-weighted EER grading scale. Database was analysed to determine the impact of numerous modifiable surgical factors on PTA-ABG after statistical accounting for underlying EER score. Results: 1679 cases performed by 11 consultant otologic surgeons were included with a mean follow up of 33.6 months. 7 significant ear risk factors independently impacted PTA-ABG on multivariable regression. These factors were weighted into a novel EER grading scale with four distinct risk groupings--each grouping associated with significantly different PTA-ABG than all other groupings (all p<0.001). EER correlated with PTA-ABG better than established MERI (p=0.003) or OOPS (p=0.032) grading scales. Conclusions: The EER grading scale is a statistically-valid tool that helps predict ossiculoplasty hearing outcomes while providing context for interpretation. Understanding and utilizing the EER grading scale also gives important insight into optimization of surgical technique and materials.