Your Okidata dot matrix printer produces the image on the paper with the printhead’s pins striking the ink onto the paper. If the printout from your printer is missing horizontal lines then you need a new printhead. Printheads for Okidata Microline printers need no tools to replace and are considered an end user replaceable item.
After replacing your printhead don’t risk damaging your new printhead by using low quality compatible ribbons. The printhead gets its lubrication from the ribbon. Use only genuine Okidata ribbons which are formulated to provided the proper lubrication to your printer’s printhead.