The rest should be done

num-converter/WebForm1.aspx

@@ -48,6 +48,8 @@
                         <asp:ListItem Value="0">Select An Option</asp:ListItem>
                         <asp:ListItem Value="1">Decimal</asp:ListItem>
                         <asp:ListItem Value="2">Binary</asp:ListItem>
+                        <asp:ListItem Value="3">Octal</asp:ListItem>
+                        <asp:ListItem Value="4">Hexadecimal</asp:ListItem>
                     </asp:DropDownList>
                 </td>
                 <td class="auto-style2">

num-converter/WebForm1.aspx.cs

@@ -21,6 +21,7 @@ namespace num_converter
             int DecimalNum;
             int BinaryNum;
             int OctalNum;
+            string hexVal = string.Empty;
             int Base1 = 1;
             //dec to binary
             if (ddlSourcePosNum.SelectedIndex == 1 && ddlTargetPosNum.SelectedIndex == 2)
@@ -46,14 +47,15 @@ namespace num_converter
                     DecimalNum /= 8;
                 }
 
-                lblResult.Text =  "Oo" + Result;
+                lblResult.Text =  Result;
 
             }
             //dec to hex
             if (ddlSourcePosNum.SelectedIndex == 1 && ddlTargetPosNum.SelectedIndex == 4)
             {
-                DecimalNum = Convert.ToInt32(txtInput.Text);
+               DecimalNum = Convert.ToInt32(txtInput.Text);
-                while (DecimalNum > 0)
+               hexVal = DecimalNum.ToString("X");
+                /*while (DecimalNum > 0)
                 {
                     Remainder = DecimalNum % 16;
                     string hexDigit = (Remainder < 10)
@@ -61,9 +63,9 @@ namespace num_converter
                         : ((char)('A' + Remainder - 10)).ToString();
                     Result = hexDigit + Result;
                     DecimalNum /= 16;
-                }
+                }*/
-
+                Result = hexVal;
-                lblResult.Text = "0x" + Result;
+                lblResult.Text = Result;
             }
             //bin to dec
             if (ddlSourcePosNum.SelectedIndex == 2 && ddlTargetPosNum.SelectedIndex == 1)
@@ -85,7 +87,7 @@ namespace num_converter
                     Result = Remainder.ToString() + Result;
                 }
 
-                lblResult.Text = "Oo" + Result;
+                lblResult.Text = Result;
             }
             //bin to hex
             if (ddlSourcePosNum.SelectedIndex == 2 && ddlTargetPosNum.SelectedIndex == 4)
@@ -113,6 +115,61 @@ namespace num_converter
                 lblResult.Text = "0x" + Result;*/
 
             }
+
+            //octal to decimal
+            if (ddlSourcePosNum.SelectedIndex == 3 && ddlTargetPosNum.SelectedIndex == 1)
+            {
+                OctalNum = Convert.ToInt32(txtInput.Text, 8);
+                Result = Convert.ToString(OctalNum);
+                lblResult.Text = Result;
+            }
+
+            if (ddlSourcePosNum.SelectedIndex == 3 && ddlTargetPosNum.SelectedIndex == 2)
+            {
+                DecimalNum = Convert.ToInt32(txtInput.Text, 8);
+                
+                while (DecimalNum > 0)
+                {
+                    Remainder = DecimalNum % 2;
+                    DecimalNum /= 2;
+                    Result = Remainder.ToString() + Result;
+                }
+                lblResult.Text = Result;
+            }
+
+            if (ddlSourcePosNum.SelectedIndex == 3 && ddlTargetPosNum.SelectedIndex == 4)
+            {
+                DecimalNum = Convert.ToInt32(txtInput.Text, 8);
+                hexVal = DecimalNum.ToString("X");
+                Result = hexVal;
+                lblResult.Text = Result;
+            }
+
+            if (ddlSourcePosNum.SelectedIndex == 4 && ddlTargetPosNum.SelectedIndex == 1)
+            {
+                hexVal = txtInput.Text;
+                DecimalNum = Convert.ToInt32(hexVal, 16);
+                Result = Convert.ToString(DecimalNum);
+                lblResult.Text = Result;
+            }
+
+            if (ddlSourcePosNum.SelectedIndex == 4 && ddlTargetPosNum.SelectedIndex == 2)
+            {
+                hexVal = txtInput.Text;
+                DecimalNum = Convert.ToInt32(hexVal, 16);
+                Result = Convert.ToString(DecimalNum, 2);
+                lblResult.Text = Result;
+            }
+
+            if (ddlSourcePosNum.SelectedIndex == 4 && ddlTargetPosNum.SelectedIndex == 3)
+            {
+                hexVal = txtInput.Text;
+                DecimalNum = Convert.ToInt32(hexVal, 16);
+                Result = Convert.ToString(DecimalNum, 8);
+                lblResult.Text = Result;
+            }
+
+
         }
 
         protected void btnClear_Click(object sender, EventArgs e)