Apple - Can I disable auto-correction of a single word on OS X?

Yes you can do it.

1. Go to System Preferences > Keyboard > Text.
2. Click the + sign on the bottom left to add a new text replacement.
3. In the Replace column, put the word you want OS X to stop autocorrecting (no spaces are allowed).
4. In the With column, put the exact same word you put in the Replace column.

Voila. You're done.

This has worked flawlessly for me every time.

When typing the word, right-click it and choose Spelling & Grammar → Show Spelling and Grammar. This will bring up a window similar to the one below. It should say that the word was not found in the spelling dictionary.

Click Learn to teach the word to the current dictionary.

                   

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The file ~/Library/Spelling/LocalDictionary only contains some of your corrections. This depends on which language you have currently selected. For example, my ~/Library/Spelling/ folder contains multiple custom dictionaries depending on the language that I have currently set. As well as LocalDictionary, I have files named en and en_GB. Editing these files instead of LocalDictionary can help set custom words when they refuse to be read from LocalDictionary.

~/Library/Spelling
├── LocalDictionary
├── dynamic-counts.dat
├── dynamic-text.dat
├── en
└── en_GB 
0 directories, 5 files

If you hold the option key when you hit the space bar after typing the word autocorrect will not change the word.