BACKGROUND In order to minimise the risk of blood transfusion errors, all healthcare professionals who participate in the transfusion process are required to be assessed as competent. New and innovative methods of training and competency assessment are required to improve the training process. OBJECTIVE The aim of this study was to explore the use of objective structured clinical examination (OSCE) in the assessment of competencies for blood administration. METHODS A mixed-methods approach was employed using structured observations of simulated practice through a three station OSCE to assess three stages of the blood transfusion process: (i) communication; (ii) documentation and (iii) identification of patients and blood products. Nurses and midwives were assessed using a 28-item checklist that was rated on a 5-point Bondy scale. After the OSCE, a questionnaire was given to participants to evaluate their attitudes. METHODS Eighty four midwives and nurses from 10 different clinical areas in a District General Hospital in London. RESULTS Inter-rater reliability between assessors was high (>0.8). Overall assessors scored participants highest on aseptic technique (97%) and lowest for positive patient identification (81%). Participants felt that the OSCE was a useful intervention and could contribute to improving the competences of staff in blood transfusion safety. CONCLUSIONS A simulation OSCE is valid and reliable for competency asessment in blood administration. The checklist is simple to complete and can be used to identify weaknesses and evaluate learning needs of staff in blood administration.