Opal is a re-occurring character in Zappa's work.
She makes her first appearance in "Lonesome Cowboy Burt" from 200 Motels, where she has a job as a waitress. Burt (played by Jimmy Carl Black) is a redneck that makes "bold advances" on her. Quotes from that scene in the film appear again in the song "Teen-age Wind" from You Are What You Is. One of those quotes is Burt's line "Opal, you hot little bitch!"
Opal appears again in the 1984 musical Thing-Fish, where she is the wife of Quentin Robert DeNameland. Both she and her husband are unfaithful to each other: he has an affair with Artificial Rhonda and she is "in de next room drinkin' Jack Daniels 'n puttin' de hurts on some ignint bell-boy".
Opal is the name of the devil in 200 Motels, so this idea relates to Titties and Beer, and to the Crossroads Legend. He is trying to get Burt to sign the contract in blood. He says "Opal you Hot little bitch, you can take this pin and hang it in your ass. You ain't the devil.." in "Teenage Wind".