With online DDL an ALTER statement is just a single statement being replicated by single SQL thread, so if a non-blocking ALTER takes 1h on master, then the slave(s) will be applying the same ALTER for another 1h, and replication will be blocked during that time. While pt-osc tool does altering a table by copying rows in chunks, and does monitor the slaves in the same time and pauses the process if necessary.
Btw. some additional info here: http://www.mysqlperformanceblog.com/2013/07/05/schema-changes-whats-new-in-mysql-5-6/
