Final answer:
Occidental's contract with a national oil company disrupted the dominance of the seven sisters and signaled a shift in the oil industry.
Step-by-step explanation:
Occidental's contract with a national oil company had a significant effect on the seven sisters, which refers to the seven major oil companies that dominated the global oil industry from the mid-1940s to the 1970s. The contract challenged the dominance of the seven sisters by introducing a new player into the market. This increased competition and disrupted the power dynamics of the industry. It also signaled a shift in global oil production and control.