Variable value assignment using RETURNING clause
Solution 1:
You need to use the INTO clause in the RETURNING to set the value being returned into your variable:
DECLARE myid OAMENI.id%TYPE;
INSERT INTO oameni
VALUES
(default,'lol')
RETURNING id INTO myid;
You also need to specify the data type of your variable; I'm glad to see postgresql supports %TYPE and %ROWTYPE.
Solution 2:
Adding to the main answer, it's worth noting that if you are doing this outside a stored procedure, you must wrap the code in a "DO" block, like so:
DO $$
DECLARE
myid mytable.id%TYPE;
BEGIN
INSERT INTO mytable (...)
VALUES (...)
RETURNING id INTO myid;
END $$