The answer, at least on 11.2, is "It depends":

This create is rolled back:

create trigger trig_foo after create on schema
begin
  raise_application_error(-20001, 'Dont do it!');
end;
/
--
create table foo as select level as id from dual connect by level<=10000;
/*
SQL Error: ORA-00604: error occurred at recursive SQL level 1
ORA-20001: Dont do it!
*/
--
select count(*) from foo;
/*
SQL Error: ORA-00942: table or view does not exist
*/

But this truncate is not:

create table foo as select level as id from dual connect by level<=10000;
--
create trigger trig_foo after truncate on schema
begin
  raise_application_error(-20001, 'Dont do it!');
end;
/
--
truncate table foo;
/*
SQL Error: ORA-00604: error occurred at recursive SQL level 1
ORA-20001: Dont do it!
*/
select count(*) from foo;
/*
COUNT(*)               
---------------------- 
0                  
*/

The answer, at least on 11.2, is "It depends":

This create is rolled back:

create trigger trig_foo after create on schema
begin
  raise_application_error(-20001, 'Dont do it!');
end;
/
--
create table foo as select level as id from dual connect by level<=10000;
/*
SQL Error: ORA-00604: error occurred at recursive SQL level 1
ORA-20001: Dont do it!
*/
--
select count(*) from foo;
/*
SQL Error: ORA-00942: table or view does not exist
*/

But this truncate is not:

create table foo as select level as id from dual connect by level<=10000;
--
create trigger trig_foo after truncate on schema
begin
  raise_application_error(-20001, 'Dont do it!');
end;
/
--
truncate table foo;
/*
SQL Error: ORA-00604: error occurred at recursive SQL level 1
ORA-20001: Dont do it!
*/
select count(*) from foo;
/*
COUNT(*)               
---------------------- 
0                  
*/