I believe it depends on the series the character belongs to. OCs are characters that are based on a series (such as sonic the hedgehog, or my little pony) but are not officially created to that series, or characters that don't belong to any series.
For instances, loona is tagged belonging to a popular made series. She isn't an OC because of her reference to that series.
But littlepip is a fan made character even those she belongs to a series fallout equestria, because fallout equestria is recognized as a fan series based on an existing series (MLP) and not its own series.