ROC stands for Republic of China, in other words Taiwan by its official name — South Korea is ROK.
Edit: To answer the other question, about the word "of", abbreviations/acronyms/initialisms are basically just words in their own right. Capitalization and punctuation, whether small words like "the" or "of" get their own letters, whether the letters are said by themselves or as syllables, or whether you pull something particularly wild like "H2G2" or "i18n", et cetera, are all kind of arbitrary and based on various constraints like euphony, brevity, uniqueness, memorability, legibility, and relatability to the original name — and then once one particular form takes off then that's the form people stick with because that's the form other people recognize.
There's probably been a decent amount of scholarly linguistics research on how exactly acronym formation works, so if you want to know more I guess you can look into that. I'm not very familiar with this topic so I'd rather avoid talking out my ass if I can help it.
Edit 2: You'll be especially aware of the fact that acronyms/initialisms are literally just words if you speak Norwegian. Earlier today I saw an ad in Norwegian that bragged about having "AI" integrated... AI is short, of course, for kunstig intelligens. I should note that I have an ASD diagnosis, short for autisme･spekter･forstyrrelser, and I am a citizen of USA, whose full name is... De forente stater.
Acronym culture can vary a lot between different languages. Japanese acronyms are especially fun. Russian acronyms tend to be pronounced "as words" even when an English speaking brain would find it far more natural to read the letters individually. Arabic you might've heard "doesn't use acronyms", even though this isn't really true, it's just that acronyms work differently in Arabic, "Hamas" is for instance an acronym of "Islamic Resistance Movement" that happens to spell out the Arabic word for "enthusiasm".