DBMS Output doesn't display while PL/SQL executes because the DBMS buffer is not released by Oracle until the PL/SQL block completes.

If you are not seeing the DBMS Output after the block executes, make sure that before you run the block, you toggle DBMS Output on via the DBMS Output tab in the SQL Editor, or:  

Toad for Oracle Version 11: Go to Options | Editor | Execute/Compile and select "Poll for DBMS output when detected".

Earlier Versions:  Go to Options → SQL Editor and turn on "Automatically poll for DBMS Output"

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