I'm only going to answer the question about visiting Germany. The status of your Oman visa is off topic here.
The good news is that in theory you can visit Germany for business purposes on a standard Schengen visitor visa. Business purposes include interviewing for a job (but not of course doing a job).
The bad news is that to convince Germany to grant you a visa (which I will assume you need - if your citizenship allows you to travel visa-free to Germany nothing I say here applies) you need to convince them of two things: 1) that you have the financial means to support yourself while in Germany and 2) that you will leave the country when you have done what you came to Germany for.
I will assume that you have the financial means to travel to Germany, stay there for the duration of the interview and leave again. If you don't (and you can't persuade the company asking for the interview to pay all expenses) then there is little point in applying.
The second is much more of a problem. You currently have no job in Oman (and I presume no family). You aren't sure if you have a valid visa to stay there. From the point of view of the immigration officer you have very little reason to return to Oman or anywhere else. It's not even clear if you would be allowed to return to Oman. They will assume you are likely to 'go underground' after you arrive in Germany, and this will probably mean they refuse to give you a visa. Not being able to show that you have a current employer is a major part of this problem.
There are things you can do to make this better. If you can prove that you are legally allowed to return to Oman (by getting your visa status sorted out), and show that you have ties to Oman (such as having a job or family there) you improve your chances. You could attempt to show that you have ties to another country, which you would return to afterwards, but this will be difficult since by living in Oman you have demonstrated that you have few ties to other countries.