Learn Spanish (Latin America): Ojalá ganáramos el premio y lo compartieras conmigo. - If only we won the prize and you shared it with me.

Because this expresses an unreal or highly unlikely wish. If you used the present subjunctive (Ojalá ganemos...), it would mean you think it is a realistic possibility, like "Hopefully we win...". By using the imperfect subjunctive, it translates closer to "If only we won...", implying it is a long shot.

Why does ganáramos have an accent mark, but compartieras doesn't?

In the imperfect subjunctive, the nosotros (we) form always gets a written accent mark on the vowel right before the -ramos ending. This keeps the spoken stress on the correct syllable. The tú (you) form (compartieras) naturally stresses the second-to-last syllable, so it does not need a written accent.

Could I say Ojalá que ganáramos... instead?

Yes! You can use either ojalá or ojalá que here. Adding que does not change the meaning at all. It is just a matter of personal preference, though ojalá que is very common in everyday spoken Spanish.

Why does lo come before compartieras? Can I attach it to the end of the verb instead?