who or whom

Going by the rule that "who" is used for a grammatical subject, where a nominative pronoun such as I or he would be appropriate, and "whom" is used as the object of a verb or preposition, then the preposition "on" would lead to "whom".

Also, only pronouns that are objects end in the letter "m" (whom, him, them).

The correct usage of who and whom, is often ignored in speech and informal writing when the word "whom" would sound forced or unnatural.

Your coach's email seems pretty informal, so I wouldn't sweat it too much.

Even formally "who" is acceptable if it is the subject of the understood clause "who will be our opponent."

on reflection, flyboy, I'm with you.

Dear Contrex and Flyboy,

Thanks very much again for your help.

That's one that sometimes gets me too. Thanks!