Move Data From Old Tables Into New?

I have to move data from old tables into new tables. I use a sqlplus script and execute multiple 'insert from select'. I thought the SQL statements are executed in sequence, one after the other. I run into this problem: Let's take this script for an example

insert into A (a,b,c,....
select a,b,c......
from X;
insert into B (a,b,....
select a,b,....
from A joined with Y;
commit;

After executing the script in our development DB

select count (*) from A
returns 200

When the script is executed in production, A table record count is 5

select count (*) from A joined with Y; (used in the second insert)

returns 200.

It seems that the two statements are executed at the same time and table A is not fully populated when the select in statement two is executed.I suspect this is due to parallel execution in production.

Will executing

ALTER SESSION DISABLE PARALLEL DML;
ALTER SESSION DISABLE PARALLEL QUERY ;