Implementing an ordered tree

Hello,

I have a Category entity. Each Category has zero or more subcategories, 
which are ordered. There is a root Category (called main category) 
which has no parent.
I have created this EO model:

oid
oidParentCategory
name
ordinalNumber

parentCategory (oidParentCategory --> oid)
subcategories (oid -->> oidParentCategory)


I need some hints on how to implement the following operations 
(methods) with "ordered way":

-add a category
-delete a category
-cut & paste a category (and its subcategories) from one level to 
another level
-reorder categories (move a category in its level)
etc.

My big headache is manipulating the ordinalNumber attribute in "EO way".


Regards,
Karoly