Can I get married at the courthouse in onslow,nc

You can get married in the Magistrate's office.

From the Register of Deeds (Onslow County, NC) page on Marriage Licenses:

9. If you are getting married in the Magistrate's office, you will need $20 cash and two witnesses. The Magistrate's office is open 24 hours a day 7 days a week. No appointment is needed.

Here is the link to the page with more information on the requirements for marrying in NC:
http://www.onslowcountync.gov/departmentnopb100.aspx?id=22817

The clerk at the Magistrate office will do the wedding. The cost is 20 dollars, ask them when you go in and get the marriage license.
Register of Deeds - Marriage License Requirements