When you call document.write after a page has loaded it will eliminate all content and replace it with the parameter you provide. Instead use DOM methods to add content, for example:
var OpenWindow = window.open('mypage.html','_blank','width=335,height=330,resizable=1');
var text = document.createTextNode('hi');
OpenWindow.document.body.appendChild(text);
If you want to use jQuery you get some better APIs to deal with. For example:
var OpenWindow = window.open('mypage.html','_blank','width=335,height=330,resizable=1');
$(OpenWindow.document.body).append('<p>hi</p>');
If you need the code to run after the new window's DOM is ready try:
var OpenWindow = window.open('mypage.html','_blank','width=335,height=330,resizable=1');
$(OpenWindow.document.body).ready(function() {
$(OpenWindow.document.body).append('<p>hi</p>');
});