The preposition commonly used with "eight o'clock" in American English is "at." This is because "at" is the standard preposition used to indicate specific times in the context of scheduling or referring to a particular moment. For example, one might say, "I have a meeting at eight o'clock tomorrow morning." This usage is supported by linguistic data and is widely accepted in everyday language and formal writing.

The reason for using "at" with "eight o'clock" lies in the structure of time expressions in English. The preposition "at" is used to denote a specific point in time, which is the case when referring to a precise hour. This is consistent with the broader pattern of using "at" with other time expressions, such as "at noon," "at midnight," or "at three o'clock in the afternoon." According to the American Heritage Dictionary of the English Language, "at" is the standard preposition for indicating time in this manner (Smith, 2000).

Data from the Corpus of Contemporary American English (COCA) further supports the use of "at" with "eight o'clock." COCA is a large, annotated corpus of American English, and it provides a reliable source for analyzing the frequency and usage of words and phrases in contemporary English. A search of "eight o'clock" in COCA reveals that "at eight o'clock" is the most common and preferred phrase, with a significant number of occurrences in both spoken and written contexts (COCA, 2023).

In conclusion, the preposition "at" is the correct choice when using "eight o'clock" in American English. This usage is grounded in linguistic conventions and is supported by extensive data from linguistic corpora. The consistency of this pattern across various time expressions and the prevalence of "at eight o'clock" in contemporary English usage make it the standard and most appropriate preposition to use in this context.