Used when a female child or child-like character has notably adult-sized breasts, but otherwise retains their child-like proportions. As a good rule of thumb, use the number of heads/breasts tall the character is to decide if they rank as an oppai_loli or not (how many?). Also note that some oppai_loli pics still need the loli tag, but not all.
If you are not sure whether or not a post should be tagged as loli, ask in the loli/shota check thread.
Note that despite using Japanese words, this is an english term, in Japanese these characters are typically called "loli kyonyuu".