How can I combine multiple rows into a comma-delimited list in Oracle?

The WM_CONCAT function (if included in your database, pre Oracle 11.2) or LISTAGG (starting Oracle 11.2) should do the trick nicely. For example, this gets a comma-delimited list of the table names in your schema:

select listagg(table_name, ', ') within group (order by table_name) 
  from user_tables;

or

select wm_concat(table_name) 
  from user_tables;

More details/options

Link to documentation

Here is a simple way without stragg or creating a function.

create table countries ( country_name varchar2 (100));

insert into countries values ('Albania');

insert into countries values ('Andorra');

insert into countries values ('Antigua');


SELECT SUBSTR (SYS_CONNECT_BY_PATH (country_name , ','), 2) csv
      FROM (SELECT country_name , ROW_NUMBER () OVER (ORDER BY country_name ) rn,
                   COUNT (*) OVER () cnt
              FROM countries)
     WHERE rn = cnt
START WITH rn = 1
CONNECT BY rn = PRIOR rn + 1;

CSV                                                                             
--------------------------
Albania,Andorra,Antigua                                                         

1 row selected.

As others have mentioned, if you are on 11g R2 or greater, you can now use listagg which is much simpler.

select listagg(country_name,', ') within group(order by country_name) csv
  from countries;

CSV                                                                             
--------------------------
Albania, Andorra, Antigua

1 row selected.

For Oracle you can use LISTAGG