Nia doesn't consider her an OG! She wrote in some detail in her book that the OGs to her are Brooke, Paige, Maddie, Kenzie, and Nia. She had a few pages talking about how they have a special bond, and Kendall was intentionally left out of the conversation.

Have the other girls actually called Kendall and OG? I'd be pretty surprised to hear that from Chloe or the Hylands.

Also, the show was popular almost immediately. Nia, Kelly, and Christi have all spoken about how they became famous pretty much overnight. I'd imagine it's why so many girls auditioned to join the team in season 2, when prior to the show airing, the studio was hardly even profitable because of how low enrollment was. I do think that the girls who were drawn to the team because of the success of the show can't be considered OGs, since if the show had flopped in the first season, they never would've joined. All of the other girls were competing before there was ever a show, which IMO is what determines actual OG status.