Locking order in cursors (Oracle)
The order in which individual rows will be locked is undefined and certainly not something you should rely on since it will depend on the query plan.  

In general, I would expect that the rows would not be locked in order-- the optimizer would generally expect it to be more efficient to access all the data unordered and then apply the ORDER BY.  If there was a composite index on pk_field, field1, field2, I could see the optimizer potentially choosing to do an ordered scan of the index and then locking each row in the table but that seems unlikely.