The Ossia Tool is pretty much useless (even for one-measure ossias). You need to add another staff, change the size (to 75%?), add it to the parts you want to see it (using Manage Parts) and hide the measures (using cutaway staff style) where you want the staff hidden.

Hi Adrian,

thanks for your fast reply. I noticed that the Ossia tool is not that helpful.

The key word in your answer is "Mange Parts". 

Would you please specify (step by step if possible) how to make a few measures from the new added part appear in an older part?

Thanks again and greets.

Benny

Hi again,

please do not bother- I found the way to merge 2 parts. Thanks a lot again.

The Ossia Tool does have certain advantages, but it's use is very specific.

One advantage is how braces/brackets are scaled. If you resize a a group of staves using the percent tool, the brace does not get resized proportionally—it remains at the 100% staff size. However, a brace generated using the Ossia Tool does get resized according to the percentage of the staff. This allows you to adjust the space between the staves without worrying about a distorted brace.

Another advantage is being able to place clef and signature information before the barline.

A third advantage of the Ossia Tool is that it can work for partial measure regions by adjusting the margins and offsets.

In the ossia below, the notes were entered on a second grand staff with staff lines hidden via staff styles. I used the Ossia Tool to draw the staff lines, clef. key signature, and brace from an empty measure (thus, not drawing any notes).