Hi Jean.
My Latin's a little rusty, but I think it is in memoriam...it's certainly not in memorium! That's not even a Latin phrase at all.
Here's why:
[warning! lingua-geek tirade follows; if you don't know what cases are most of this won't make sense]
in = into (accusative) or in (ablative)
memoria = feminine noun, singular, "memory" or "remembrance"
in memoria + am (accusative) = in memoriam = "into remembrance"
as opposed to
in + memoria + [long a] (ablative) = "in remembrance"
Clearly the accusative case is the standard for this sort of thing, as with Alfred Lord Tennyson's poem "IN MEMORIAM A. H. H." This is appropriate since the accusative case implies motion, and here we're wishing Oakeshott's name into the annals of history, not just stating that it's allready there.
See
http://www.math.ohio-state.edu/~econrad/lang/ln1.html
http://www.informalmusic.com/latinsoc/prep.html
http://charon.sfsu.edu/tennyson/inmemoriam.html
Thanks,
Jake
(Who was a linguist by trade, once upon a time)
Sen. Free Scholar
ARMA Deputy Director