Purpose: Long-term opioid therapy (LTOT) has been shown to be associated with opioid overdose, but the definition of LTOT varies widely across studies. We use a rigorous LTOT definition to examine risk of opioid overdose by duration of treatment. Methods: Data were from a large private health insurance provider in North Carolina linked to mortality records from 2006-2018. Eligible patients were adults (18-64) newly initiating opioid therapy after a pain diagnosis or surgery. We defined LTOT as ≥1 opioid prescription per month totaling ≥60 days’ supply within 90 days. We used inverse probability- (IP) weighted cumulative incidence functions to estimate three-year risk of opioid overdose and IP-weighted Fine-Gray models to estimate subdistribution hazard ratios, comparing LTOT to short- to medium-term opioid therapy (SMTOT). We also examined modification by derived indication of acute pain or surgery versus chronic pain. Results: We identified 491,369 patients, and 1.7% were exposed to LTOT. The three-year risk of opioid overdose was 0.3 percentage points (RD w= 0.003, 95% CI: 0.001, 0.005) higher in LTOT patients compared to patients with SMTOT. The weighted hazard of opioid overdose was 4.4 times as high (HR w 4.42, 95% CI 2.41, 8.11) among patients exposed to LTOT versus SMTOT. We did not find meaningful modification by clinical indication for opioid therapy. Conclusions: Exposure to LTOT was associated with increased risk of opioid overdose in this population of privately insured patients using a rigorous definition of LTOT. These findings confirm the importance of guidelines to minimize duration of opioid therapy whenever possible.