Spray the crock of your slow cooker with non-stick cooking spray.
Season both sides of the chicken with salt and pepper; place chicken in an even layer in the slow cooker.
Top chicken with garlic and mushrooms.
Pour Marsala and sherry into the crock.
Cover and cook for 5 hours on low.
When ready to serve, melt butter in a medium sauce pan.
Add oregano to melted butter and stir for 30 seconds.
Whisk in flour to form a roux, about 30 seconds.
Remove chicken from slow cooker and transfer liquid to the saucepan with the roux.
Whisk until thickened slightly, about 1-2 minutes.
Add additional salt and pepper if needed.
Serve sauce over chicken and mushrooms.