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.
Turns out pillow requires zlib and jpeg. You need to install zlib-dev "opkg install zlib-dev". I was unable to find jpeg. I assume there's a way to do that but I haven't ventured far enough into this to be able to try that. You can get around the jpeg requirement by disabling it in the installation arguments.