Yeah, I got caught with this today, if you use the 64bit version you get a Segmentation Fault 11
Process: sqlplus 
Version: ??? (???)
Code Type: X86-64 (Native)
Parent Process: bash 
PlugIn Path: /Library/Oracle/*/libclntsh.dylib.10.1
PlugIn Identifier: libclntsh.dylib.10.1
PlugIn Version: ??? (???)
Date/Time: 2011-07-24 16:59:56.281 -0500
OS Version: Mac OS X 10.7 (11A511)
Report Version: 9
Crashed Thread: 0 Dispatch queue: com.apple.main-thread
Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x0000000000000000
VM Regions Near 0:
must use 32bit version (modified from bradleyran posting)
1: create a new folder in /Library and renamed it Oracle
2: inside that folder, I created 3 more folders named "lib","java", and "network"
3: Inside "network" make an "admin" folder to house your tnsnames.ora file.
4: copy the .jar files from the basic client folder to the java folder.
5: copy all the other libraries (ie contain dylibs) from the basic client folder to the "lib" folder
6: copy the "sqlplus" file to the /usr/bin/ folder
7: If you use the bash shell, edit the /etc/profile (ie sudo vi /etc/profile) and set your environment variables, ie:
8: test /usr/bin/sqlplus username/password@serverdnsname
Oracle software get's build upon installation and the last working MacOSX was 10.5 Leopard and Oracle 10g, both are obsolete. There are some howto's and blogs that will show you how to install Oracle under 10.6 but it's all based on compiling the software under 10.5 or copying old binaries - it's all not worth the hassle. Forget about doing this under 10.7 Lion.
The general answer for all Oracle needs under MacOSX Intel is Oracle Virtualbox. It's free and actually a lot better than installing Oracle or related tools under OSX. You can install Virtualbox and install a supported OS (guest) that will run side by side with your primary Mac system (host) or use the pre-build VM's available at http://www.oracle.com/technetwork/community/developer-vm/index.html. There is no noticeable performance penalty and you have another supported system in your machine without having to modify your host operating system or fiddle around with the Oracle installation.
If you all you need is SQL access to Oracle under MacOSX, use SQL Deverloper, which will also work under 10.7 Lion. It is available for free from Oracle OTN.
That doesn't help for when any kind of database clients need to be used. Oracle bindings for languages like Ruby, PHP, and Perl are broken in 64-bit environments on OS X. The issue is not running the Oracle DB, the issue is accessing it.
The question of Oracle database access will depend on interface requirements. IMO, running a virtual machine is currently the only feasible solution for Oracle database development under MacOSX. Oracle database support looked promissing when 10g was released, but not anymore. I guess Apple is an inconvenient platform for Oracle since future OS versions are not very predictable and often lack backward compatibility. Oracle would have to do a special Apple only product since MacOSX is neither just only a user interface nor a standard Unix platform that Oracle supports.
I use Apple since the early 90's and beside a few recent negative developments, I don't see a more compelling or better alternative desktop platform. However, I use Oracle free VirtualBox and Oracle Linux for all Oracle related tasks. Using a Virtual Machine for testing and development is more convenient anyway.