Many people have told me that playing a brass instrument will seriously mess up your tone quality and embouchere on the oboe. Is it true? This is because I really want to play trumpet in marching band.

well, last school year the other oboist, as well as a flutist, played melophone during marching band. it didn`t really mess either of them up ... if you want to play trumpet, I suggest practicing oboe the same amount as the trumpet so as not to ruin your embouchure or anything else about your playing technique.

I would highly not suggest the trumpet......the embouchure on the Oboe does get ruined. I know a girl who picked the Saxophone and also played the Oboe and the embouchure of the Oboe was seriously affected.