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.