Add "Equation" to the category drop down box at:
Tools - Options - OpenOffice.org Writer - AutoCaption
Using "add caption" on an equation is problematic due to the size of the equation. However, a manual equation number can be inserted by:
Insert - Fields - Other - Variables - Number Range - Equation
The "Value" field needs to read "Equation + 1".
The equation can then be normally referenced via:
Insert - Cross reference - Equation - Equation i - Numbering