From Wikipedia:
Sesame Street premiered on November 10, 1969. The very first scene was a clay-animated sequence showing two creates forming the words "Sesame Street", followed by the opening theme music. The first regular performer to appear on screen is Gordon (Matt Robinson) who introduces one of his students to his wife, Susan, as well as Bob and Mr. Hooper. Big Bird also appears, though the first puppet-style Muppets to appear on screen were Ernie and Bert.
During this first season, Big Bird and Oscar the Grouch were the only Muppets to regularly appear in Street scenes, while Ernie and Bert and others debuted in separate segments. Big Bird had a much more naïve voice and a smaller head with fewer feathers, and Oscar was a "toxic" orange...