1. FreeBSD：
FreeBSD是FreeBSD项目提供的一个免费的开源操作系统。它是基于两个类似Unix的操作系统Berkeley Software Distribution(BSD)和Research Unix设计的。它与Linux操作系统非常相似。它是较早的著名操作系统之一，但是现在与其他操作系统相比，它的使用率有所降低。 FreeBSD的第一个版本于1993年发布。FreeBSD的最新稳定版本是12.1。
2. OpenVMS：
OpenVMS是由Digital Equipment Corporation(DEC)开发的多用户和多处理操作系统。它是基于虚拟内存的操作系统，旨在用于分时，批处理和事务处理。它可用于许多平台，包括服务器和工作站。它是使用BLISS，VAX Macro，C，Ada，PL / I，Fortran，UIL，Pascal，SDL，C++和DCL编写的。 OpenVMS的第一个版本于1977年发布。此操作系统中使用的内核与模块是整体的。
OpenVMS和FreeBSD之间的区别：
| OpenVMS | FreeBSD |
|---|---|
| It was developed by Digital Equipment Corporation (DEC). | It was developed and is owned by The FreeBSD Project. |
| It was launched in 1977. | It was launched in 1993. |
| Its target system types are server and workstation. | Its target system type is server, workstation, NAS and embedded systems. |
| Computer architectures supported by OpenVMS are VAX, Alpha and IA-64. | Computer architectures supported are IA-32, x86-64, ARM, MIPS and PowerPC. |
| Its kernel type is Monolithic with modules. | Kernel type is also Monolithic with modules. |
| Its native APIs are Proprietary. | The native APIs are BSD/POSIX. |
| Its package management is PCSI or VMSINSTAL. | Package management is Port collections and packages. |
| It has the preferred license of Proprietary. | Preferred license is BSD. |
| File systems supported by OpenVMS are Files-11, ISO 9660, NFS and CIFS. | File systems supported are UFS2, ZFS, ext2, ext3, FAT, ISO 9660, UDF and NFS. |