Here are some nuances of the word:
* Familiarity: It's typically used for women you know, or even strangers you feel comfortable with. It's not as formal as "おばあさん" (obaasan, grandmother).
* Respect: While it's familiar, it's still a respectful term. It's not considered rude or disrespectful to use it, even if you're not related to the woman.
* Regional Variations: In some regions, it can be used for women who are not necessarily elderly, but still older than the speaker.
It's important to note that "obachan" can be interpreted differently depending on context and relationship.