This is my first post so apologies if I've done something wrong. Any comments and corrections are very welcome.

As requested, the ungolfed version:

string ug(int n) { 

        // The sting we'll be returning
        string ret = ""; 

        // The width and height of the output
        int s = n * 2 + 1; 

        // for loop for width and height
        for (int height = 0; height < s; height++) 
        {
            for (int width = 0; width < s; width++) 
            {
                // Matches on top-left to bottom-right diagonal line
                if (height == width) 
                {
                    // If this is the center, write the 'sun'
                    if (width == n) 
                    {
                        ret += "O"; 
                    }
                    // If this is not the center, add the diagonal line character
                    else 
                    {
                        ret += "\\"; 
                    }
                }
                // Matches on top-right to bottom-left diagonal line
                else if (width == s - height - 1) 
                { 
                    ret += "/";
                }
                // Matches to add the center line
                else if (width == n) 
                { 
                    ret += "|";
                }
                // Matches to add the horizontal line
                else if (height == n) 
                { 
                    ret += "-";
                }
                // Matches all others
                else 
                { 
                    ret += " "; 
                } 
            } 
            // Add a newline to separate each line
            ret += "\n"; 
        } 
        return ret; 
    }

This is my first post so apologies if I've done something wrong. Any comments and corrections are very welcome.

C#, 230 bytes

string g(int n){string r="";int s=(n*2)+1;for(int h=0;h<s;h++){for(int w=0;w<s;w++){if(h==w){if(w==n){r+="O";}else{r+="\\";}}else if(w==(s-h)-1){r+="/";}else if(w==n){r+="|";}else if(h==n){r+="-";}else{r+=" ";}}r+="\n";}return r;}

This is my first post so apologies if I've done something wrong. Any comments and corrections are very welcome.

C#, 230 226 bytes

string g(int n){string r="";int s=n*2+1;for(int h=0;h<s;h++){for(int w=0;w<s;w++){if(h==w){if(w==n){r+="O";}else{r+="\\";}}else if(w==s-h-1){r+="/";}else if(w==n){r+="|";}else if(h==n){r+="-";}else{r+=" ";}}r+="\n";}return r;}

This is my first post so apologies if I've done something wrong. Any comments and corrections are very welcome.

C#, 230 bytes

string g(int n){string r="";int s=(n*2)+1;for(int h=0;h<s;h++){for(int w=0;w<s;w++){if(h==w){if(w==n){r+="O";}else{r+="\\";}}else if(w==(s-h)-1){r+="/";}else if(w==n){r+="|";}else if(h==n){r+="-";}else{r+=" ";}}r+="\n";}return r;}

This is my first post so apologies if I've done something wrong. Any C&C very welcome.

C#, 230 bytes

string g(int n){string r="";int s=(n*2)+1;for(int h=0;h<s;h++){for(int w=0;w<s;w++){if(h==w){if(w==n){r+="O";}else{r+="\\";}}else if(w==(s-h)-1){r+="/";}else if(w==n){r+="|";}else if(h==n){r+="-";}else{r+=" ";}}r+="\n";}return r;}

This is my first post so apologies if I've done something wrong. Any comments and corrections are very welcome.