The OCF antenna is a bad idea, prone to enhance the local noise, and also the signals from the nearby MW stations. You may have almost no local noise from neighbors, but your line is carrying the noise from your own house to the antenna, and as the antenna is unsymetrical, the noise go back with wanted signals to your receiver.
The simplest way to get by : changed your OCF in a center fed antenna with both arms same height above ground, and run the line in the center plane as far as possible from the antenna, as least a quater wavelenght for any band of interest, so the common mode noise on the line is just coupled equally to both arms of the antenna, and as the arms are connected in opposition of phase to the line, the noise is cancelled. BTW, the strong MW signals are also cancelled, as the antenna becomes insensitive to vertically polarized waves on lower frequencies.
The OCF is just a concept for transmitting in the harmonically related ham bands. Nothing good for receiving, as the SWR is not a problem, specially if you use a high impedance antenna input! You even have no necessity to use coax cable. Any twisted line is enough. Use a twisted line from any scrap of phone or network cable to go trough your window sill. The outdoor part of the line can be made with widely spaced wires (lower losses when the whole antenna+line is tuned), with just plastic spacers cut in plates for flower boxes (or any plastic good enough for sunlight exposure). The impedance of such a line is not important. For the best results, you need an ATU including a balanced input.
A center-fed antenna with tuned line is known as 'doublet' in England and 'Levy antenna' in France and elsewhere. A doublet apparently sized for 6MHz can give good results on any frequency between between 3 and 12 MHz, and even more. Lower, the tuning becomes very sharp, higher the receiving pattern has more and more lobes. I don't understand why that antenna is so badly understood in the USA.