Open whichever program you would like to work in (Writer, Calc, Impress, Math, Draw or Base).
From the main screen of the program, you can press "F11" or click on "Format" in the toolbar at the top of the screen. Then click on "Styles and Formatting" to access the Styles menu.
From the Styles menu you can pick a particular style to apply to your selection. You can also create your own style by clicking on "Custom" and formatting the text any way you want. The style can be saved for future use. This is useful if you submit journal articles to multiple publishers, for instance, because you can easily reformat your documents based on the requirements of each particular journal.
Calc is the only OpenOffice program that has an existing "comma style" in the format menu; it is used to format numbers larger than 999. The feature is most easily accessed by typing in the data and then selecting cells with info in them, clicking on "Format" and "Cells." The menu offers several choices for formatting numbers.