An OD intervention is any process used in an attempt to improve/change/shift the organizational culture. For example, say you are needing to implement some change. You could try to understand what the current attitudes are towards that issue throughout the organization. Then develop a plan to help people understand the change. This might involve communication, training, internal marketing, appointing change champions and so forth. All these actions that you take are interventions. You should regularly assess how your interventions are impacting and modify accordingly.