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Background There is no consensus regarding the impact of oncoplastic surgery (OPS) on rates of re‐excision and conversion to mastectomy following breast‐conserving surgery (BCS). Here these two outcomes after BCS and OPS were compared in a nationwide population‐based setting. Methods In Denmark, all OPS is registered and categorized into volume displacement, volume reduction or volume replacement. Patients who underwent BCS or OPS between 2012 and 2018 were selected from the Danish Breast Cancer Group database. Multivariable analyses were performed to adjust for confounders, and propensity score matching to limit potential confounding by indication bias. Results A total of 13 185 patients (72·5 per cent) underwent BCS and 5003 (27·5 per cent) OPS. Volume displacement was used in 4171 patients (83·4 per cent), volume reduction in 679 (13·6 per cent) and volume replacement in 153 (3·1 per cent). Re‐excision rates were 15·6 and 14·1 per cent after BCS and OPS respectively. After adjusting for confounders, patients were less likely to have a re‐excision following OPS than BCS (odds ratio (OR) 0·80, 95 per cent c.i. 0·72 to 0·88), specifically after volume displacement and reduction. The rate of conversion to mastectomy was similar after OPS and BCS (3·2 versus 3·7 per cent; P = 0·105), but with a lower risk in adjusted analysis (OR 0·69, 0·58 to 0·84), specifically after volume displacement and reduction procedures. Findings were similar after propensity score matching. Conclusion A modest decrease in re‐excision rate and less frequent conversion to mastectomy were observed after OPS compared with BCS., Compared with breast‐conserving surgery, oncoplastic surgery results in a modest decrease in re‐excision rates and less frequent conversion to mastectomy. Rates similar to breast‐conserving surgery only