The OneAgent installer does not require Java on your system, nor does it deploy or use Java for the operation of the OneAgent (ignoring that OneAgent may be used to monitor your Java applications).
For Linux and Windows as two common platforms for OneAgent, you will find in our Dynatrace Help the following pages that'll describe more about what the OneAgent installer performs as changes to your system:
Linux, see What happens during installation? section
Windows, see What happens during installation? section
From a security standpoint, there may be particular permission required at installation time, so please review the security related topics:
For other supported platforms, please have a look at this section of Dynatrace Help to locate the OneAgent topics related to the platform you wish to monitor.