Currently I hit M or O first while watching TV to get to the tV listings.

Is there a way to do it quickly?

yes, you need to setup a jump point.

To do this go to your mythtvs website, clik on settings, click on key bindings. Assign a key combo to the "Program guide"

This should work havn't tried it myself.

Just be carefull not to use a key that is used for somthing else.