Earlier, if someone used an apostrophe instead of an 'okina DLHawaiian disallowed it, but I think they figured out some platforms don't have an easy way to type 'okina for those who want to try not using the word tiles, so it seems that they accept apostrophes now (thanks DL)
I assume you mean difference in sounds. If you hear the word spoken in isolation there's not really a difference that I can hear. However in reality you almost never say a single word alone, and all Hawaiian words end with a vowel. So if you say the word 'Okina by itself out just sounds like "o-ki-na". But if you say "ka 'okina" the letter would be clearly heard.
If you mean difference in meaning, the are two completely different words. 'Okina is a letter.