In these cases, from a Unicode and OpenType (and hence HarfBuzz) point of view, those are not really ligatures. Ie. fonts will not tell HarfBuzz to automatically convert 'oe' into 'œ'. To Unicode, 'œ' is a standalone character. Now, fonts *can* tell HarfBuzz to substitute such a ligaure, they can even say do it under French only. But if it's not mechanically decidable, then it shouldn't be in the font.
That said, the substitution mechanism in OpenType allows for contextual matches. So, if, say, a ligature should be formed only if followed by a certain string of characters, that can be expressed in the font.