Sebastian,
download the Omnia-related files here: https://api.turris.cz/openwrt-repo/omnia/
This server contains all the goodies - packages, toolchain and OS images.
What you’re looking for is the SDK: OpenWrt-SDK-mvebu_gcc-4.8-linaro_musl-1.1.15_eabi.Linux-x86_64.tar.bz2 This contains the compiler and OS-related files.
See this document: https://oldwiki.archive.openwrt.org/doc/howto/obtain.firmware.sdk
This info has been mentioned here before, useful thread: Cross Compile - Howto?