This was with me actually installing pysnmp on the omega with "pip install pysnmp", so it was trying to compile it on the omega and I'm guessing even if I did "pip install pycrpytodomex" it would have had the same results.

installing gcc, after uncommenting the base and packages lines for openwrt in /etc/opkg/distfeeds.conf, seemed to do the trick. Though, I ran into another problem. It ran out of free space, not on my sd card, but on the /tmp mount. I'm not sure how to increase that, or if I really need to, but for anybody else that might come across this the solution was to specify a build folder for pip like this.

pip install --build /root/tmp pysnmp