The aim was to analyse the association between severity of complications up to 30 days after surgery and pre-operative nutritional and physical performance parameters. The participants were a subsample of the previously published PERATECS study (ClinicalTrials.gov: NCT01278537) and included 517 onco-geriatric patients aged ≥ 65 years, undergoing thoracoabdominal, gynaecological, or urological surgery. Post-operative complications were classified according to the Clavien Classification System (CCS). Independent risk factors related to the severity of complications, defined as major complications (CCS IIIa-V) and graded complications (CCS grade 0-V), were analysed using logistic and ordinal regression, respectively. In total, 132 patients suffered major post-operative complications. The development of major post-operative complications was independently associated with body mass index (BMI) < 20 kg/m2 , hypoalbuminaemia (< 30 g/l), longer duration of surgery, and specific tumour sites (upper gastrointestinal, gynaecological, colorectal) (all P < 0.05). Higher-grade complications were predicted by Timed Up and Go (TUG) > 20 s, hypoalbuminaemia (< 30 g/l), higher American Society of Anesthesiologists (ASA) status III-IV, longer duration of surgery (> 165 min), and specific tumour sites (upper gastrointestinal, gynaecological) (all P < 0.05). Mini Nutritional Assessment (MNA) scores and weight loss were not independent risk factors for the severity of complications. Nutritional and physical performance risk factors that predicted the severity of complications differed between major and higher-grade post-operative complications, but hypoalbuminaemia independently predicted both. The results support the need for pre-operative risk screening. Due to the explorative nature of the study, further research is required in larger cohorts to corroborate these findings.