There are several ways to avoid the Oppo needing to know anything about ALAC files.
1) Convert ALAC to FLAC permanently. Many ways to do this and ALAC and FLAC are so similar, the conversion process is VERY quick. Some Media Player software can do this, but there are also some standalone apps for converting 1 file format to another format and some of those are free.
2) Play music on computer using nice media player interface with album art available, playlist support... all the stuff that makes computer audio interesting. The Media Player software will convert ALAC (or any other compatible format) to PCM and send the PCM to the Oppo via the USB DAC input (you need a USB cable with a type A connector on one end and a Type B connector on the other end. The computer does all the work and the Oppo plays the music without having to know anything about ALAC.
3) Same as #2 but you send the music over a network... Ethernet being much more likely to provide seamless playback than wireless which can be problematic with music if it is not "n" speed or faster and if there's not much other activity on the wireless network that would steal bandwidth from the music playback activity.
Many media player software apps for the PC will play ALAC files fine from a PC so you don't even have to have a Mac with Media Player software if you don't already have one. There are also file converter programs for PC that will convert ALAC to FLAC quickly. There are free media player apps for the PC... Foobar2000 is probably the best one for sonics, but JRiver Media Center for $50 is quite impressive, especially if you appreciate interesting graphical user interfaces with 3D album art modes for selecting music to listen to, etc. For the Mac, Decibel at $33 has a very basic user interface with excellent sonics. I avoid iTunes like the plague for music playback as it doesn't bypass any of the computer's internal audio system and is among the worst-sounding ways to listen to music.