Final answer:
O'Brien did not write about the Proles; instead, it was Winston Smith in George Orwell's 1984 who believed the Proles were the key to revolution. Marxist theory supports the idea of a proletariat-led revolution, adapted by others like Mao Zedong.
Step-by-step explanation:
O'Brien did not write in his diary that any hope of revolution against the Party must come from the Proles; however, this is a misconception often associated with the character Winston Smith in George Orwell's novel 1984. Winston himself believed that the only hope for overthrowing the Party would be through the Proles. Your question may have confused Winston's thoughts with those attributed to O'Brien.
In the context of Marxist theory and historical application, it is noted that Karl Marx believed the proletariat would lead the revolution, a sentiment that has been adapted in different ways by revolutionary leaders like Mao Zedong, who included other social strata for the revolutionary cause in China.