I think we're certainly headed in that direction:
The 400 wealthiest individuals on the Forbes 400 list have more wealth than the bottom 150 million Americans---and their wealth is growing, while that of those at the bottom is shrinking.
In 2010, 25 of the 100 largest U.S. companies paid their CEO more than they paid in U.S. taxes. 15 Fortune 500 companies paid no taxes at all in 2014.
There are 22 corporate lobbyists for every member of Congress. 50% of members of Congress become lobbyists when they leave office.
If an oligarchy is the concentration of wealth and power among a small group of people, I think we're well on our way.