How to close a modal popup with Ok button?

A modal window is not a pop-up window. Popup window means another browser window is launched and it has it's own html DOM. Otherwise, it's just a routine locator on the only browser that is up. In case of another popup, since there are 2 windows (2 DOM's), the system has to know which window you want to interact with. In Subject7, locator definition you can specify the popup window which is a text that exist on the target window and doesn't exist on the other. If you want to close a popup window you can use the command CLOSE_POPUP. 

Hope this helps. 

Please right click on the OK button and see if you can inspect it. If not, it is an alert box, and you can use the Alert OK command to close it, otherwise the instructions provided by Rex above should hold for a true pop up.