Can you reserve seats to Austria when you're in Italy?

You should read the rules of you pass, but tickets can be purchased up to 15 minutes before the train departure so you should be able to reserve a seat. There are tons of seats, they aren't capacity controlled like in France.

There is an option for pass holders to reserve seats on-line. Look for the train you are taking on www.trenitalia.com/tcom-en, then select Global Pass as type of fare.

Your question isn't completely clear.

Are you wanting to reserve seats in Austria or on a train from Italy to Austria?

Either way, yes, on line, and if the latter, in a station. Just the company (OeBB or Trenitalia) will change and you need to use the correct website or app.

Thank you all very much! I was worried that there might not be enough seats.

@nigel, yes I meant from Rome to Austria.

Yup, so it shouldn't be a problem. Easiest with the interwebs but easy in station too, although stations come with queues and questionable "helpers (for a tip)" hanging around the machines. Those trains to Austria can get fairly busy so if you could do it an extra day earlier that might be good....

If you are looking for sleeping compartment or couchette for the Nightjet to Vienna, you should buy that ASAP as they do sell out.