An Order to Show Cause is a hearing wherein you explain to the judge why you have not been able to file the Proof of Service of Summons. It sounds like you have made reasonable efforts to do so, and therefore, you just need to appear in court to explain what you have done thus far.
Hopefully, you can have everything corrected prior to the OSC. However, even so, you will probably still have to appear for the OSC hearing, at which time the court should "discharge" the OSC.
The information presented here is general in nature and is not intended, nor should be construed, as legal advice. This posting does not create any attorney-client relationship with the author (who is only admitted to practice law in the State of California). For specific advice about your particular situation, consult your own attorney.
Make sure that whenever you file something with the clerk's office or the court, that you take an extra copy for them to stamp or "conform" that way you will have proof of your own that you filed the document.
Disclaimer: The materials provided below are informational and should not be relied upon as legal advice.
It may well be that as your proof of service was being filed, the court clerk issued the OSC notice. You should call the clerk of the Department to which your case is filed and ask them to confirm this was the case. As my colleagues stated, it is always a good idea to obtain a conformed copy. If the OSC is not taken off calendar, be sure to appear before the court that day and have your conformed copy to show it to the court. To be more cautious, you may consider preparing a declaration that explains you timely had complaint served, and attach as an exhibit your conformed proof of service, and file your declaration with the court in advance of the OSC hearing. Be sure to consult your own attorney to protect your legal rights.