An OPMS primarily consumes disk space in /var (specifically /var/log), /opt/asm/var/www/assets and, /tmp. The files in /var should be manually removed on a periodic basis. The files in /opt/asm/var/www/assets and /tmp, are purged regularly by cron jobs started when the OPMS is installed.
Focusing on /var specifically. ASM logging should be kept in the following directories...
It is safe to delete any log file not currently being written to. For example, if /var/log/optunnel contains....
...it is safe to delete everything but the optunnel-client.log.0 log.
As for /opt/asm/var/www/assets and /tmp. In OPMS versions below 8.5, there are known issues in which assets are not correctly deleted from /opt/asm/var/www/assets. In this case, it is safe to delete any file older than 2 weeks in both locations (please note these files will be in sub directories such as tmp/rbtm1/opt/asm/var/www/assets/). In this case, /tmp serves as temporary storage for the assets and /opt/asm/var/www/assets is permanent storage until the data is removed (normally by a cron job).
Finally, the command "apt-get clean" on Debian or "yum clean packages" on RHEL/ CentOS can be run post OPMS install. These commands remove all packages from the local cache directory, freeing disk space. However, these packages will re-download if needed to install a program in the future.