I do not quite get the "similarities", I'm afraid, because although it is assumed that Rastapopoulos is of Greek origin his (legal) businesses are more in the realm of media rather than oil, gambling or shipping as you describe for Onassis.
In "Read Sea Sharks", the Emir describes di Gorgonzola (Rastapopoulos) as being involved in newspapers, radio, film and TV (more Rupert Murdoch than Onassis :).
On the other hand, Onassis was well-known for his luxury yacht on which he would entertain leading celebrities, politicians and businessmen much like Rastapopoulos does in "Sharks", so there is a connection there.
Speaking of "Sharks", we see the return of Muller as Mull Pasha. Muller first appeared in "Black Island" and it is believed that in that story he was based on Georg Bell, an adventurer who was involved in a German Nazi plot to destabilise Soviet Russia by flooding the country with fake roubles (see here for more details
). In "Sharks", Muller is based on Glubb Pasha aka John Glubb
, a British soldier who led the Jordanian army of King Hussein I during and after World War II.
It has also been suggested that Cartoffoli, the reckless Italian driver in "Calculus Affair", was based on the pianist Arturo Benedetti Michelangeli
, well-known for his passion for fast cars.