Upvote:3
If you enter the USA on any status other that your F-1 student visa (such as your ESTA), there is a high chance that your SEVIS record and F-1 student status will be terminated. This will prevent you from being able to work on your OPT once it is approved.
The most simple next step is to wait until your OPT is approved and have your university or a trusted friend mail your new EAD to you in Austria. If you are unable to wait in Austria, you should contact the international student center of your school/university to get their advice on entering the US after your program completion date while you are still waiting to receive your EAD.