Background and Objectives: Individuals with heroin-related opioid use disorder are at high risk of poor oral health and impaired oral health-related quality of life (OHRQoL). However, data on the determinants of OHRQoL in this population remain limited. This study aimed to assess oral health status and OHRQoL and to identify key predictors of OHRQoL among hospitalized patients with heroin-related opioid use disorder. Materials and Methods: A cross-sectional study was conducted among 97 hospitalized patients with heroin-related opioid use disorder. Data were collected through structured interviews and clinical oral examinations, including the Decayed, Missing, and Filled Teeth (DMFT) index and the Simplified Oral Hygiene Index (OHI-S). OHRQoL was assessed using the Oral Health Impact Profile-14 (OHIP-14). Spearman correlation and multiple linear regression analyses were performed to examine associations and identify predictors of OHRQoL. Results: Participants had poor oral health, with high DMFT (19.1 ± 6.1) and OHI-S (2.0 ± 1.2) scores. OHRQoL was markedly impaired, with a mean OHIP-14 score of 29.6 ± 7.0, particularly in the psychological and social domains. OHIP-14 scores were strongly correlated with DMFT (r = 0.754, p < 0.001) and OHI-S (r = 0.552, p < 0.001). Longer duration of heroin use was associated with higher DMFT scores, poorer oral hygiene, and greater OHRQoL impairment. Multiple linear regression analysis identified unstable housing/homelessness (β = 0.166, p = 0.012), DMFT (β = 0.624, p < 0.001), and OHI-S (β = 0.272, p = 0.007) as significant predictors of poorer OHRQoL. Conclusions: Individuals with heroin-related opioid use disorder experienced substantial oral health burden and impaired OHRQoL, strongly associated with caries experience, poor oral hygiene, and housing instability. These findings highlight the need to integrate prevention-oriented dental care into addiction treatment services and to address the broader social determinants of oral health.