SMS comparison problem

Projects Programming
10 
    String SmS [7];
    String Command [7];
    Command[1] = "O";
    Command[2] = "N";
    int i = 0;
    
   for(;i<7;i++)
   {
      SmS[i] = sms.read();
   }
   if ( (strcmp(SmS,Command)))
   {
    Serial.println("\nOutput now ON");
   }

ive used this now but there is a ERROR saying: "cannot convert 'arduino::String*' to 'const char*' for argument '1' to 'int strcmp(const char*, const char*)'"
and btw could you tell me the diffrence between strcmp() and strncmp()? thx