You could use the DATE function:
SELECT col1, col2, ..., coln
FROM order_table
WHERE date(order_date) = '2012-05-03'
But this is more efficient, if your table is large and you have an index on order date:
SELECT col1, col2, ..., coln
FROM order_table
WHERE order_date >= '2012-05-03'
AND order_date < '2012-05-04'