I don't know if there is a better way, but here is how I do it:

mypage.aspx:
Code:
<%@ Page Language="C#" AutoEventWireup="true" %>
<%@ Import Namespace="System" %>
<%@ Import Namespace="System.Web" %>
<%@ Import Namespace="System.Data.OracleClient" %>
<script runat="server" src="oralib.cs"></script>
...