No its not possible, as pfsense/opnsense doesn't use plain logfiles, small quote from their documentation:
pfSense uses a Circular Log format known as clog to maintain a constant log size.
There are multiple benefits to this method. The primary advantage is that the log files cannot grow and fill up filesystems. The downside is that they are no longer plain text logs, and cannot be directly used by programs like cat or grep. If they are viewed directly, "random" binary data may be seen inside the log and entries may appear out of order.
So the only way would be hosting a log stash myself and forwarding to the ovh ones right?