To handle LIMIT and OFFSET within Oracle DBs you have to use some magic. There is a Blog post, which describes the general procedure. Doctrine uses this ROWNUM stuff too, it is implemented within OraclePlatform.php. Doctrine needs two integer values for LIMIT and OFFSET. But you can also set both to NULL, the methods of the Query class (setMaxResults() and setFirstResult()) accept NULL values too.
If you think, it is a good idea to send PHP_INT_MAX as default for LIMIT, it would be a fail. Within the OraclePlatform.php Doctrine must add the given OFFSET (maybe 20) to the LIMIT (maybe PHP_INT_MAX), so you will run into a datatype overflow. This will result into PHP_INT_MIN, a very large negative number. The resulting SQL would try to filter your ResultSet from OFFSET (20) to a large negative number, it would be empty always (except you will set the OFFSET to 0).
Also, if you set a large LIMIT to force getting all records from a query, you force Doctrine to wrap your query with some of the ROWNUM stuff, which results in a more complex query and an increased query time. Let LIMIT = null, if you need all records. Only set OFFSET > 0 (or != null), if you need the next page of the results.