To have a pagebreak in each before each subdocument in OO's master document

"After setting up the pages, or before, edit the Styles and Formatting

(Menu > Format > Styles and Formatting or F11 ).

Right-click Heading 1 and select Modify...
Select the Text Flow tab
Check Insert in the Breaks section
Select Page for Type
Select Before for Position
Click OK

Each Heading 1 will now have a page break inserted just before."