I did that:
You can select separate measurement units for OpenOffice.org Writer, OpenOffice.org Writer/Web, OpenOffice.org Calc, OpenOffice.org Impress and OpenOffice.org Draw documents.
1.Open a document of the type for which you want to change the measurement units.
2.Choose Tools - Options.
3.In the left pane of the dialogue box, double-click the application for which you want to select the measurement unit.
Double-click OpenOffice.org Writer if you want to select the measurement units for text documents.
4.Click on General.
5.On the General tab page, select the measurement unit. Close the dialogue box with OK.
... from the help file.
That would be the option one above from that what blackbird showed.
This might be a workaround - if somewhat clumsy:
For example, in a text document, open Format - Paragraph - Indents & Spacing. To indent the current paragraph by one inch, enter 1 in or 1" into the "Before text" box. To indent the paragraph by 1 cm, enter 1 cm into the input box.