Running Mac OSX Lion, trying to bundle install a project that included the ruby-oci8 gem, I received the following error:

Building native extensions.  This could take a while...
ERROR:  Error installing ruby-oci8:
	ERROR: Failed to build gem native extension.

The fix:

Fortunately, the 64-bit client for Oracle has now been updated for Lion/Mountain Lion. Download them from Instant Client Downloads for Mac OSX (Intel x86). Downloading “Basic Lite” and the “SDK” under Version 11.2… (64-bit) worked for me.

Unzip the files to ~/instantclient_11_2 so that the sdk directory and libs are directory beneath it:

$ ls ~/instantclient_11_2 
BASIC_LITE_README    genezi               libclntsh.dylib.11.1 libocci.dylib.11.1   libocijdbc11.dylib   ojdbc6.jar           uidrvci
adrci                libclntsh.dylib      libnnz11.dylib       libociicus.dylib     ojdbc5.jar           sdk                  xstreams.jar

Add the following to my .zlogin or .bash_login

export DYLD_LIBRARY_PATH=~/instantclient_11_2

Rerun the login shell (exec $SHELL -l) or restart your shell.

Link the libclntsh.dylib. in ~/instantclient_11_2 to a versionless lib file name:

ln -s libclntsh.dylib.11.1 libclntsh.dylib

Your ruby-oci8 should install now.