Background Oseltamivir is usually not often prescribed (or reimbursed) for non-high-risk patients consulting for influenza-like-illness (ILI) in primary care in Europe. We aimed to evaluate the cost-effectiveness of adding oseltamivir to usual primary care in adults/adolescents (13 years +) and children with ILI during seasonal influenza epidemics, using data collected in an open-label, multi-season, randomised controlled trial of oseltamivir in 15 European countries. Methods Direct and indirect cost estimates were based on patient reported resource use and official country-specific unit costs. Health-Related Quality of Life was assessed by EQ-5D questionnaires. Costs and quality adjusted life-years (QALY) were bootstrapped (N = 10,000) to estimate incremental cost-effectiveness ratios (ICER), from both the healthcare payers' and the societal perspectives, with uncertainty expressed through probabilistic sensitivity analysis and expected value for perfect information (EVPI) analysis. Additionally, scenario (self-reported spending), comorbidities subgroup and country-specific analyses were performed. Results The healthcare payers' expected ICERs of oseltamivir were euro22,459 per QALY gained in adults/adolescents and euro13,001 in children. From the societal perspective, oseltamivir was cost-saving in adults/adolescents, but the ICER is euro8,344 in children. Large uncertainties were observed in subgroups with comorbidities, especially for children. The expected ICERs and extent of decision uncertainty varied between countries (EVPI ranged euro1-euro35 per patient). Conclusion Adding oseltamivir to primary usual care in Europe is likely to be cost-effective for treating adults/adolescents and children with ILI from the healthcare payers' perspective (if willingness-to-pay per QALY gained > euro22,459) and cost-saving in adults/adolescents from a societal perspective.