As much The Hag is a total legend in country music, I was about as surprised as you were to learn that his 1973 single, "If We Make It Through December" was his only country hit that crossed over to the mainstream pop chart (specifically, the Top 40 of the US Billboard Hot 100) during his long and illustrious career. "Okie from Muskogee" barely missed the Top 40 (it ranked #41).
Of course, on the country charts, he had approximately 35 number one hits and he was inducted into the Country Music Hall of Fame, the Nashville Songwriters Hall of Fame, and he received the Grammy Lifetime Achievement Award. That just goes to show that just because you're a one-hit-wonder doesn't mean you necessarily lack talent. Sometimes it just means you're more of a niche artist rather than a mainstream pop artist. In this case, The Hag's niche was country music and he totally owned it.