The deal makes Occidental larger but "significantly less valuable," analyst Doug Terreson wrote.
Occidental shares dropped 4% to $45.24 on the New York Stock Exchange after Terreson cut his rating on the stock to in line from outperform.
He said the company's pledge for greater capital discipline and enhanced corporate governance would likely prove "fleeting."
Terreson said the deal "destroyed value" and is 30% dilutive to return on capital employed. Occidental completed its $55 billion acquisition of Anadarko last week.
A potential tie-up was revealed in April and a deal announced in May. Anadarko's board approved Occidental's bid over a rival offer from Chevron (CVX - Get Report) and agreed to sell some African assets to France's Total (TOT - Get Report) .
The downgrade, which did not include a stock-price target, was offset by BofA/Merrill Lynch, which on Monday raised its 12-month price target on Occidental's stock to $80 from $78.