your use of double-quotes is correct. if
//span[text()="Yes, It's OK"]
doesn't match, it might be because the xpath engine has a lowercase bug (i have encountered PHP+DOMXPath+libxml2 systems where it would only match lowercase text even for text with uppercase characters, never quite figured out the problem there), or it might be because the text has hidden whitespace you're not aware of, maybe it actually has a hard-to-spot whitespace at the start or the end? anyway maybe contains() will work:
//span[contains(text(),"Yes, It's OK"]
or.. if it has the lowercase-issue i have encountered in the wild once before, maybe this will work:
//span[contains(text(),"yes, it's ok"]
(that really shouldn't be required, but i have been in that situation once before, and that was years ago, and that was... probably php5)