Order of onload events

If I have the page with IFRAME and the document loaded into IFRAME is on 
onload calling a script in the main page (the page containing the 
IFRAME), will this script be already accessible? (Well, typically the 
script will be parsed before IFRAME tag, but I don't know if I can take 
it for sure that onload on IFRAMEd document will always succeed.)

  Thanks,
    JZ