Alright, so originally, I'm an oboe player, and I have been playing oboe for 5 years now. But I will be joining marching band tomorrow (well, my first practice is tomorrow) and obviously, I can't play oboe, so my mom is having me switch to mellophone. Is the mellophone a huge change from oboe? It's in bass clef, and I'm so used to treble... But, by band teacher says most oboe players switch to the mellophone. Am I going to be okay switching to this instrument when I don't even know how to play it?

WHAT ?!:!?! !
The mellophone is nothing like the oboe. And the mellophone reads in treble clef, at least it did when I was in band.
I do not think the mellophone will affect your oboe playing in a negative way. As a horn player I also played oboe and english horn in college for fun and I sounded just fine.

Fingerings are pretty simple.

The oboe players at my school play alto saxaphone in marching band, but melophone is way better.

I don't like having oboe players or basson players marching with a single reed instrument. The reason I do not like it is because the player can get into the habit of using their teeth on the single red mouthpiece and then inside if they get nervous can revert back to that and BAM, there goes another reed; cracked because we bit on it.

I like the idea of horn, or mellophone. You are still pushing your lips together for the embouchre to make the sound and the horn/mellophone mouthpiece is smaller than the other brass instruments. It also has more resistance than the other brass so you get the backpressure feeling a little bit.