If O joined the firm before M, then this means that H, N, K, and J also have to go before M since H and N are before O and K and J are before M. This means the earliest M could have joined was 1966.
It's important to remember that both O and M each have two people before them, so if O is before M, then there are four additional people in addition to O that must come before M.
Does this make sense? Let us know if you have any other questions!