Even though they alternated in the first season, often throughout an episode, there are quite a few scenes in the first season where frank Oz voices Miss Piggy even though other Frank Oz characters speak in the same scene (and in the same take, so it's not like the filming was stopped and then a close-up was shot, allowing frank oz to switch characters).
At the end of the Jim Nabors episode, after Scooter says, "i'm shocked!", Fozzie says "i'm fozzie", miss piggy says "i'm piggy", and Animal says "i'm Animal", and Piggy is voiced by Frank Oz in this scene. Of course, Richard Hunt was performing Scooter, and one of them had to dub Miss Piggys voice.
In the u.k. spot from the Peter Ustinov episode, Miss Piggy led the song "somebody done sombody wrogn song", and Fozzie had a few solo lines. Both were voiced by Frank, but the voices in songs were always recorded before they were filmed, so I guess it didn't really matter who was performing Piggy.
During Fozzies joke in the opening of the Kaye Ballard episode, Miss Piggy reacts to the set-up of Fozzies joke, and both are performed by Frank Oz (I think Frank did the actual performing of Fozzie in that scene).