NOTATIO

maybe an odd question but...
is there some sort of rule about what goes into an ossia staff?

generally speaking, do you put the more difficult to perform option in the ossia staff? or the easier option in the ossia?

or does it not really matter?

as in, the one the composer actually prefers to hear goes in the main staff, while a "less desirable" - regardless of difficulty - option goes in the ossia?

Not odd at all. Generally, the more difficult version is the one the composer prefers and goes on the main staff, so the ossia represents a facilitation or an alternate for some other reason, like range.

However, there are a few cases where this is not true. Perhaps the most well-known exception is the ossia to the cadenza in the first movement of Rachmaninoff's Piano Concerto no. 3. In this case the more difficult version is shown as an ossia, perhaps because Rachmaninoff performed (and apparently preferred) the simpler version in the main text.

I t think there might also be some instances in the piano works of Liszt where an outlandishly difficult passage is given as an ossia.

Another case just occurred to me. In the Brahms Paganini Variations Book 2 Var. 8 the ossia represents an alternate of equal importance to the main text (and printed below the main text in the original edition!). This allows the player to play it as a variation on the repeat of each section if desired, or to replace the main text completely.

very cool, and interesting examples.
thank-you, John.

For me the ossia represents the second class version by the composer. The main version is usually the preferred one, both musically and in the context.
And I agree with John's observations above. Berg's Violin Concerto has ossia as an easier, but less appealing version for the soloist.