Adherence to continuous positive airway pressure (CPAP) treatment tends to be low. Brief validated instruments focusing on shared decision making have not been used in a CPAP context. The aim was to investigate factorial structure, categorical functioning of the response scale and differential item functioning across sub-populations of the CollaboRATE and Sure questionnaires among patients with obstructive sleep apnea (OSA) before CPAP treatment is initiated. A prospective design, including 193 objectively diagnosed (polygraphy) OSA patients (68% men, 59.7 years, SD 11.5) from two CPAP clinics was used. Data were collected with the following questionnaires; Sure, CollaboRATE, Attitudes to CPAP Inventory, Epworth sleepiness scale, minimal insomnia symptoms scale, and hospital anxiety and depression scale. Objective CPAP use was collected after 6 months; 49% demonstrated decisional conflict on SURE and 51% scored low levels of shared decision making on CollaboRATE. Unidimensionality was found for both CollaboRATE (one factor explaining 57.4%) and SURE (one factor explaining 53.7%), as well as local independence. Differential item functioning showed both to be invariant across both male and female patients. Internal consistency (Cronbach's alpha 0.83) and composite reliability (0.89) were good. Latent class analyses showed that patients with low decisional conflict and high shared decision making were more adherent to CPAP treatment. CollaboRATE and SURE provided good validity and reliability scores to measure shared decision making and decisional conflict in relation to CPAP treatment. The questionnaires can be used by healthcare personnel as a tool to simplify the assessment of shared decision making.