The estimation of operations and maintenance (O&M) costs for weapon systems has been termed 'infeasible' due to: 1) a lack of detailed prior (O&M) costs, 2) a large amount of uncertainty in the operational tempo for the system, and 3) uncertainty in the predicted reliability of system components. This research proposes the creation of a flexible discrete event simulation model to estimate O&M costs by predicting events that occur during a system's life cycle. Such a model takes as inputs a given concept of operations, maintenance strategy, and system reliabilities to determine lifecycle events such as: consumables used and maintenance operations performed on the entire system throughout its life cycle. The uncertain cost of each event can be used to estimate a distribution of total O&M costs. The results can finally be analyzed to determine the attribution of the uncertainty of those costs to all of the different possible sources.