The PHP op-code caching software or accelerator are meant to improve the performance of any PHP applications which  is also true for OpenX as given in OpenX blog entry “ Use a PHP accelerator to speed up OpenX”.
How accelerators are working: As PHP is a scripting language, PHP parser parses and compiles the lot of code for each request it receives before executing it. If there are thousands of request are coming at a time it will increase the overhead of CPU and will also increase the memory usage. On this heavy load the system may become unresponsive.
The PHP op-code caching software’s /PHP accelerators  works by caching the compiled byte code of PHP script thus removing the compulsions of parsing and compiling the PHP code for every request. Most of the PHP accelerators save that compiled code into the shared memory and directory execute the program from that shared memory. Storing the byte code into the shared memory will reduce the amount of unnecessary memory copying done at execution time.

Eventually any PHP accelerator will increase the speed of code execution by the factor of 2 to 8 times by reducing the load on server. The accelerating factor depends upon the underlying PHP execution environment   and the amount of source code executed in each request.

 Name of Some PHP Accelerators:
1)  Alternative PHP Cache (APC)
2)  eAccelerator
3)  XCache
4) Zend Optimizer +  (PHP Extension By Zend)
5) Windows Cache Extension for PHP
All above PHP accelerators will give you about 5 to 20 percent of memory gain and at least speed gain of 3X.
The APC is a PECL Package and has very small footprints with each of configuration. The APC is maintained by the core PHP developers so there is very rear chance of incompatibility or any big bug in the package.
Some versions of eAccelerator and XCache are unstable with new versions of PHP like they may give segmentation fault often in-between the execution of script, chose these accelerators after verifying the compatibility issue with the version of PHP.
The eAccelerator is marginally better than other in speed and memory utilization.
If your PHP installation is hosted on windows platform with IIS server then you can use “Windows cache extension for PHP” which will provide opcode cache, file cache, resolve file path cache and session cache.