What type of balun at the OCFD feedpoint? I suspect a 4:1, but is it a voltage or current balun? Ideally you want a current balun on an OCFD, though many ship with a voltage balun due to cost, wanting additional vertical radiation like a windom, or whatever the builder hoped to achieve.
You could try adding a current balun below the feedpoint if the antenna already has a voltage balun, or just outright replace a voltage balun with a current balun.
Adding the popular airwound coaxial balun might work, too, but note those often have very small working ranges and can even make issues worse depending upon feed characteristics. A current balun with a wide working frequency range for RF resistance is typically the better approach.