The MOP is the period of time you are required to physically occupy your HDB flat before you can sell it on the open market. When you buy an HDB flat, you are subject to a five-year MOP. During this period you are not allowed to:      1. Sell your flat (except back to HDB for special reasons)     2. Rent our your whole flat (renting rooms is okay)     3. Purchase a private property MOP begins from the date of key collection and you must be residing in your flat to fulfill the MOP.