"Show New page if ID" is blank isn't working

Greetings good helpers.

I have what appears to be simple "if else end if" conditional but it isn't working.

My code says:

subID = request("subID")
mainID = request("mainID")

Then the "If" conditional:

If subID = "" OR if mainID = "" then
  PageTitle = "Add New record"
Else
strSQL = "Select * from mytable where subID = " & subID & " and mainID = " & mainID & " "
set rs=conn.Execute(strSQL)
PageTitle = "Update Record"
end If

Then on my form, I have something like:

<form name="myform" method="post">
         <input type=hidden name=ChildId value="<%=ChildId%>">
        <input type=hidden name=Chsid value="<%=currentChsid%>">
    <table border=1 cellspacing=0 cellpadding=0 width="100%" valign=top height="370px" borderColorDark="#5C604D" borderColorLight="#5C604D">
      <tr>
        <td>
            <TABLE cellSpacing=1 cellPadding=1 width=100% border=0>
              <TR>
                    <TD width=155 style="color:#5C604D;font-weight:bold"><%=PageTile%></TD>
                    <TD Colspan=5 align=right valign=top>&nbsp;</TD>
                </TR>
                </TABLE>

and the rest of the code.

Problem is that always shows "Add new Record" even when subId and mainID are not blank.

Any help is definitely appreciated.