Masks that were open never showed the add, subtract, etc., options. Never did. They make no sense if the mask doesn't create an alpha channel. From CS6:
Stroke behavior also has not changed. Create a bunch of masks open or closed, add Effects>Generate>Stroke, select all masks and animate the start or end point and the paths will be stroked sequentially. I can't make it do anything else. It sounds like you don't have stroke set to write on transparent. This bypasses the mask's alpha channel and eliminates the need to set the mask options to none. Also, there is no need to close the path on an O if you don't want to. Most of the time when I use stroke to do a write on effect I'll use only one path because it's easier to get the timing down so that it looks like a signature. When someone writes with a pen time passes as they move from one letter to the other. I accomplish this by simply drawing a path that is pretty close to the path a pen would follow while writing the text. Overlapping parts of a letter like the letter A are handled with masks on the text layer that I animate as the stroke passes the overlap. I also use the stroke layer as a track matte instead of using Stencil Alpha because it's easier to control, edit, and modify.
Here's a screenshot of how I would set up a mask for the word Path:
And here's what the masks on the text layer would look like before I started animating the stroke:
Each mask would stay in place until the stroke needed to reveal that part of the letter. While this takes a little while to set up it's by far the best way to write on text.
You can edit the playback options in the Preview Panel. Also, there is an update due any time now that is supposed to fix many playback issues.