Hey Partial! I've gone ahead and talked to this user in St. Defoko's and resolved the issue, but for the sake of everyone else I'll publish what we did so that everyone can benefit from it. This is especially useful to other mac users.
1. Text format
The original OTO was accidentally in rich text format, which added a bunch of extra formatting symbols to the file and made UTAU unable to read it. If you open up TextEdit and it looks like this:
you're in rich text format. To switch to plain text, go to Format > Make Plain Text, or use the keyboard shortcut Command + Shift + T. This will keep all of the text and strip away things like font size, font color, etc. and make it possible for UTAU to read it.
2. Encoding
UTAU-Synth plays along really nicely with Unicode, unlike Windows UTAU. However, if you have a UTF-8 encoded oto_ini.txt file, it might open up and become complete garbage. This is because UTAU-Synth actually reads OTOs as Shift-JIS by default, for compatibility with Windows UTAU. To make sure that it's correctly reading the OTO as Unicode, make sure this line is at the very beginning of the file.
3. Mismatched reclist and OTO
For any reclist more complicated than Japanese CV, you'll often be using a base OTO instead of creating the OTO from scratch. Always be sure to double check that you're matching up the correct reclist with the correct OTO! The entries in the OTO must match the file names of your recordings exactly in order for UTAU to read the samples correctly.