The first time I ever saw Oppai Loli is Hestia from Danmachi. But, is Hestia the first Oppai Loli in anime and manga? Was there an Oppai Loli before her?
Note: this answer uses the term "loli" as defined by Japanese anime fans: it's either age-related or appearance-related (AniOta), or to the extension of either childlike face, short height, no waist, loli voice, or even deformed (pixiv).
Hestia is probably the first character that's called ロリ巨乳 (LOLI kyonyuu, big-breasted loli) in-universe, which is a famous term characterizing her physical trait, but there were many oppai loli before her.
Here are articles on AniOta Wiki (Japanese), Nico Nico Pedia (Japanese) and pixiv dictionary (Japanese) that list oppai loli characters with their traits. Here is a Google Sheets made by me for compiling a list of oppai loli characters based on those sources.
Based on AniOta Wiki, Nico Nico Pedia, pixiv Dictionary (+ some YouTube videos), Saori Kido, a 13-year old girl from Saint Seiya (December 1985) is the earliest oppai loli character.
Image courtesy of Wikipedia