Lightroom Red Mask with Brush Tool?

[AshleyCo]

Hi!  I am trying to use my brush tool in Lightroom, but when I run the cursor over the area, it doesn't show the red area where I am brushing.  Is there a setting that I can turn the red mask on while selecting the area? 

[Bob Somrak · Accepted solution]

Hit the "o" key for overlay

[Laurelf941]

I am using a free trial download.  The small case letter 'o' does not activate the red mask.  It will show up on the image in the filmstrip but that goes away when the cursor is moved.  A glitch or is there some other step I need to take.