I had to add a bass clef symbol after creating an Ossia. It defaulted to the treble clef despite the fact that the original part was in the bass clef.
Second, when I create an ossia that begins with a partial measure, I am unable to copy and paste notes at the start of the ossia. It defaults to the first full measure.
 Please ignore my sentences below - I am mixing up Ossia with Cue, sorry.
are you trying to change something in the Ossia? Ossias reflect another staff exactly. You can move the start and end of an Ossia to any point of a bar. You can also in the Property Panel define in which key you want the Ossia to be displayed. What is it exactly you can’t achieve?
 Please ignore my sentences above - I was mixing up Ossia with Cue, sorry.
I wish that this were true. I added an ossia to. bass clef staff and Dorico assumed that it was treble, adding numerous ledger lines below the staff. I changed the staff to bass and continued working. I shouldn’t have to stretch an ossia when I only need to change a few notes. I should be able to click inside the partial measure to select it and paste notes. The only option is to click the complete measure. The incomplete measure is ignored when pasting notes.
You can Alt-click notes to an ossia stave, including in a partial measure. You can double-click in the ossia staff to invoke the caret there, or move the caret from other staves using the up/down arrows, then paste. You can’t single-click on an empty partial bar because there’s no rest to select.