Lo with oler?

Hola Mabelle,

without context it is a bit difficult to see, but imagine that there are several different types of fruit in a room and you say "she smells it all". You would in English use "it" to refer to the fruit. This is the same function that "lo" has in "Ella lo huele todo."

Without this type of context, as a general sentence we would probably drop "lo" and say: "Ella huele todo" meaning "She [can] smell everything", as if saying she "has a very good sense of smell". 

I hope this clarified it.

Saludos

Inma