My default JRuby uses 1.9.3 mode. I added a .jrubyrc file to my project containing

compat.version=2.0
cext.enabled=true

Then I was able to do ruby -v and see it was in 2.0.0 mode. This verified that the .jrubyrc file was being read. I tried to install the ox gem, and got

Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.

        /Users/jeremy/.rvm/rubies/jruby-1.7.4/bin/jruby extconf.rb 
NotImplementedError: C extension support is not enabled. Pass -Xcext.enabled=true to JRuby or set JRUBY_OPTS or modify .jrubyrc to enable.

I tried closing my terminal, and re-opening, but still got the error. After I did export JRUBY_OPTS=-Xcext.enabled=true I was able to bundle the gem. So it seems the .jrubyrc file just ignored that option.

My system:
jruby 1.7.4 (2.0.0) 2013-05-16 2390d3b on Java HotSpot(TM) 64-Bit Server VM 1.6.0_51-b11-457-11M4509 [darwin-x86_64]
OSX 10.8.4