Arch Linux/Manjaro: In a terminal, run
sudo pacman -S onlyoffice-bin: https://aur.archlinux.org/packages/onlyoffice-bin/
Do not save documents with equations in .odt format. Only save them in .docx format. Otherwise, while the document will save, you will lose your equations.
Go to Insert > Equation and click on the square root symbol. It works just like Word's equation editor.