If you have spent 4 (maybe 5) rigorous years earning a degree from The Ohio State University or another esteemed institute of higher education, you have joined an elite group of members and you should be proud of this. You should also refer to your membership in this group correctly, as you just spent 4 years (and a boat load of money) to further your intellect.

Definitions:

Alum [al-uh m]
noun, Chemistry.
1. Also called potash alum, potassium alum. a crystalline solid, aluminum potassium sulfate, K 2 SO 4 ⋅Al 2 (SO 4) 3 ⋅24H 2 O, used in medicine as an astringent and styptic, in dyeing and tanning, and in many technical processes.

2. A depraved 11w member known for posting questionable GIFs in a crusade for meaningless Helmet Stickers

To be clear, calling yourself an Alum is referring to a rare chemical compound rooted in potassium.

To properly refer to your association to your institution of higher learning:

Alumnus - The masculine singular form

Alumni - The masculine plural form

Alumna - The feminine singular form

Alumnae - The feminine plural form