No. DS9 is the only series that has a main-title-credited character that is enlisted. O'Brien was made a CPO because he was a transporter "chief" throughout the first five years of TNG. His rank just carried over to DS9 when the character transferred posts--though I do believe Miles got a promotion out of the deal. It more accurately established his rank, there was no requirement for a "token" enlisted. The term "transporter chief" had fallen out of use.
In a sense, yes, the writers felt the need to have a character that "worked for a living" so to speak. They needed a guy who was more relatable to the working class viewers than the usual officers, because there is a long standing stereotypical divide between enlistees, who don't have formal college education (often earning their way up the ranks through lots of hard work and experience), and Academy/college trained Officers (who have learned everything in school).
Strictly speaking, the difference between enlisted men and officers is not their education, but whether they are given a commission from their government. Officers have a commission, enlisted men don't. In practice, it is customary to only give commissions to people with college educations. In the case of O'Brien, since he is an engineer he would likely have had a high degree of education. He was originally a minor character on TNG, before becoming a main character on DS9. They may have wanted to bring him over so that they would have an enlisted man as a main character, but in practice there really isn't any difference between him and the officers.