The first ODI match was between which countries?

The first official Test match was played on the 15 March 1877, between England and Australia at the Melbourne Cricket Ground, where Australia won by 45 runs. England won the second match (also at the MCG) by four wickets, thus drawing the series 1-1.

This was not the first ever international cricket match however, which was played between Canada and the United States, on the 24th and 25th of September 1844.