Participants will be recruited from London Health Sciences Centre in London Ontario. Patients deciding to proceed with elective cardiac surgery will be screened in the surgeon's office, pre-admission clinic or the 6-inpatient ward for eligibility to participate in the study and provided with a letter of information. Each patient will be required to provide written consent in order to participate in this study. The study will be approved by the Health Sciences Research Ethics Board at Western University. Patients who have consented to participation in the study will be seen by the research coordinator at the patient's pre-operative clinic appointment where the research coordinator will explain the purpose and nature of the study and obtain written informed consent. Also, at that time, patient demographic information will be collected, as well as baseline data of chest x-ray (CXR) and a six minute walk test (6MWT). Pre-operative 6MWTs will be performed on a standardized 30 meter track in the basement of University Hospital and will comply with the American Thoracic Society Guidelines (2002). Patients enrolled in the study will be randomized to one of two groups on the day of their surgery; OPEP treatment or sham treatment group. On the day of surgery or post-operative day (POD) 1, the research coordinator will bring the device (OPEP or sham depending on randomization) to the patient's room. The sham devices have been manufactured to be externally identical to the OPEP devices allowing for patient blinding, however the sham devices do not contain the internal mechanisms provided expiratory pressure. All patients (sham or OPEP) will be seen by a physiotherapist on the day of extubation and receive instructions on how to properly use the device, in addition to receiving standard post-operative care. Patients will be instructed to set the resistance on the device to the highest setting and perform up to 15 breaths in the sitting position at least twice per waking hour. The resistance and number of repetitions may be decreased to adjust for patient tolerance as deemed appropriate by the treating physiotherapist. Compliance will be measured through a log book completed by the patient and/or their family. Patients will be reassessed by a physiotherapist on POD #2 and #3 to ensure proper technique and compliance with the device. All patients will continue to receive standard care each day as per the clinical pathway and may receive additional cardiorespiratory PT techniques as deemed necessary by the PT. Patients will be instructed to continue with OPEP treatments (sham or OPEP) as described until POD #5. Outcome assessment will be conducted by a blinded assessor. The outcomes and timing of outcome assessment is described in the outcome measures section and will terminate on POD #7 or discharge from hospital, whichever occurs first. If patients require continuous positive airway pressure (CPAP), bilevel positive airway pressure (BiPAP), or reintubation during this period, they will be instructed to stop using their OPEP device and outcome assessment will occur only until POD#7.