ArchWike has a great openbox page. That is how I found out about Arch in the first way
Extract the desired Xcursor theme to either /usr/share/icons (system-wide access) or ~/.icons (local user access). There are also a limited amount of themes available in the community repository that can be installed using pacman.
Add this to ~/.Xdefaults:
where [name-of-cursor-theme] is the name of the cursor theme directory. For example:
To change the size: