Oracle client must be installed on a shared file system which is accessible to all the nodes where Informatica is installed. If you are planning to use the clustered file system for Informatica, then make sure to install Oracle client with in the same cluster where Informatica is part of.
Not entirely correct. You can also install the Oracle runtime client on each node separately, and many customers do that. Which one you prefer (shared installation / local installation) is a different question, but both are possible, and both have their advantages and disadvantages (shared: disk space, local: availability without network shares).
Thanks for the correction Nico. I have seen many customers installing Oracle client on a shared file system mostly due to maintenance reasons. For example adding a TNS entry on a shared file system would affect all the nodes in a multi-node system as opposed to adding them separately on each node on a local installation. Also I have seen less downtimes for Oracle client installations or upgrades.
We use this type of setup. Like PowerCenter libraries I personally recommend installing Client on local Machine and use a common TNSNAMES.ora file from a shared location.
Below Articles will explain how to setup common tnsnames.ora usi9ng TNS_ADMIN variable.