Ossia is a musical term for an alternative passage which may be played instead of the original passage. The word ossia comes from the Italian for "alternatively" and was originally spelled o sia, meaning "or be it". Ossia passages are very common in opera and solo-piano works.

What means ossia?

: or else —used as a direction in music to indicate an alternative and usually simpler form of a passage.

What does Colla parte mean in music?

'With the part', 'with the voice'. An indication to play another, written-out part, or to an accompanist to look to the main part for guidance on tempo, etc. From: colla parte in The Oxford Companion to Music »