You can write a PL/SQL function to return that cursor (or you could put that function in a package if you have more code related to this):
CREATE OR REPLACE FUNCTION get_allitems
RETURN SYS_REFCURSOR
AS
my_cursor SYS_REFCURSOR;
BEGIN
OPEN my_cursor FOR SELECT * FROM allitems;
RETURN my_cursor;
END get_allitems;
This will return the cursor.
Make sure not to put your SELECT-String into quotes in PL/SQL when possible. Putting it in strings means that it can not be checked at compile time, and that it has to be parsed whenever you use it.
If you really need to use dynamic SQL you can put your query in single quotes:
OPEN my_cursor FOR 'SELECT * FROM allitems';
This string has to be parsed whenever the function is called, which will usually be slower and hides errors in your query until runtime.
Make sure to use bind-variables where possible to avoid hard parses:
OPEN my_cursor FOR 'SELECT * FROM allitems WHERE id = :id' USING my_id;