Mine is the same as Skippytate- Van Halen opening for Black Sabbath at MSG in 1978 (not 1980 like vinyl shadow mentioned) Ozzy had already left by 1980. In fact, his first tour was opened by Def Leppard which might get an honorable mention in this category.
VH had just released their first album and my friends and I loved the album and loved the show. However, at MSG anyhow, the Sabbath fans were rabid. And by the end of the set they were more than ready for Van Halen to get off the stage. It was an incredible show. So glad I got to see Ozzy with Sabbath.