Pinkyboyid has beaten me to it but I'll ask again anyway:
What format have you scanned the document into?
However, thinking how I fill in scanned forms myself, I'm not sure that you'll be able to do it in OpenOffice Writer (or in any other word processing program, such as Microsoft Word) anyway.
I use either an image-processing program (such as Gimp) to add text onto a 'photo' of the document or a DTP program (such as Serif PagePlus SE) to import such a 'photo' onto the page and then create text fields on top of it.