The Oxford English Dictionary doesn't say "This is the meaning of the word because we say so", it says "This is the meaning of the word because we found examples that illustrate the meaning." It then cites examples, including the earliest example they could find.

The second edition of the OED cites the variant spellings "halloa" from the year 1781 and "hallo" from 1864, when Edison was 17 years old. It's first citation for "hello" is from 1883.