By the way, postgres has had INSERT ... RETURNING since v8.2 (2006). I rather suspect that Oracle's kludgy PL/SQL-based imitation was bolted on after that, and because of it. (Well, maybe 'kludgy' is too harsh. But the above disadvantages certainly make it imperfect compared with Postgres.)
Well, let's have a look.
Oracle has documented everything and the kitchen sink for ages, and the answer to almost any Oracle-related question, including far more than you ever wanted to know about Larry Ellison's coffee mug, could simply be RTFM. After about 15 sec with google and some hops through Oracle's website, I found the 8.0.4 documentation CD (also available as a ZIP file), and from there, the INSERT statement documentation. The documentation has a "copyright 1997" statement, matching the information found here and here.
Now guess what's in the INSERT statement documentation:
The RETURNING Clause
An INSERT statement with a RETURNING clause retrieves the rows inserted and stores them in PL/SQL variables or bind variables. Using a RETURNING clause in INSERT statements with a VALUES clause enables you to return column expressions, ROWIDs, and REFs and store them in output bind variables. You can also use INSERT with a RETURNING clause for views with single base tables.
That was in 1997, and around that time, I had my first contact with Oracle.
Now, according to Wikipeda, PostgreSQL was at that time somewhere between 6.0 and 6.2. I could not find PostgreSQL documentation from that time, but hopping across the versions starting at 7.1 shows no traces of RETURNING until 8.2. And 8.2 was released at the end of 2006, at least 9 years after Oracle had documented RETURNING.
So no, sorry, Oracle did not copy that from PostgreSQL. It looks like PostgreSQL copied the idea from Oracle.
Note: Not the implementation! PostgreSQL works like a simple SELECT, whereas Oracle requires explicit use of bind variables. (Update:) Both ways have their advantages: The PostgreSQL way of RETURNING mutates the INSERT into something that behaves like a SELECT with (big) side effects. This allows easy fetching of the return value, but breaks the assumtion found in many programs and libraries that only SELECT statements return data, and especially INSERT doesn't. Oracle's way keeps INSERT a strictly non-SELECT statement and uses only bind parameters to return data, in a way that would also work with PL/SQL code.
And I wish I had read the fine manual a little bit more 15 years ago, that would have simplified my software quite a bit. On the other hand, my software had to support MS SQL, and that sucked (and still sucks) so much that it would not have made a big difference.
Yet another update: Typos fixed, thanks to AnomalousMonk.
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)