From OpenOffice.org Version 1.1 on you can use the built in macro recorder. To assign a character to a key, follow these steps:

Select Tools > Macro > Record to start the recording process.
Now click Insert > Special Character and choose the character you want, then insert it and close the special character dialog.
Click on "Stop recording" in the small macro recorder dialog.
A dialog appears asking you to save the macro with a name. Be sure to save it under Standard, not in the tree for the document currently open. That way you'll be able to use the macro use it any time a OOo Writer window is open.

Now that you have recorded the macro, you'll need to assign it to a keystroke.

Select > Tools > Macro > Macros ..., then > Assign.
Now follow > Keyboard > Functions and find your macro using the Category tree. Your macro will appear in the Function window when you've found the right module.
Select a keystroke in the Shortcut Keys list, and click Modify. The key you have selected will appear in the Keys window.