Yellow Lines on every print

Replace the Toner Cartridge

This happened to me. It was caused by a bad "oki compatable" color cartridge. One time it ruined my drum. It was always the yellow color. Decide which cartridge is printing streaky. Remove it, bang it on the inside of a large plastic garbage can. Clean it. Reinstall it . If it still prints streaky throw it befor you ruin the drum. Replace it.

Run the head clean function.

It all depends on the meaning of 'not properly'.

Try creating a document with four different squares, or words, of these colours: yellow, magenta, cyan and black. If all of them print (albeit not with the correct shades) it is probably an ICM driver error. But my guess is that only one of those four colors will print badly, due to a defective color cartridge.
The OKI 5150 makes all perceivable colours using cyan, magenta, yellow and black toners, so if one of these malfunctions, the resulting colors will lack the required shade of, say, yellow, and will appear to be "wrong". The yellow colour itself, of course, would simply disappear (the "yellow" you use might actually be 95% yellow and 5% cyan; then the "yellow" square will be printed as a very light shade of cyan, while the other colors - say, 95% magenta and 5% yellow - will turn out almost exact).

Try with the color test and replace any failing cartridge; if all four (almost) pure colours do print incorrectly, it's unlikely to be a cartridge problem, and we'll have to dig deeper.

You need to replace the magenta drum. Just because there is life left; it's possible it is scratched. Also, take a cotton ball dip in alcohol and wipe the LED's off when the cover is open; those black things that are hanging down from the cover. Also, make sure your drums are installed in the proper spots (magenta in magenta slot). If the toner "rubs off" after it prints... then the fuser is defective.

You need a new black drum.

One of the drum cartridges is stuck. Test by removing them one at a time with the printer off, then turning on.

First, clean the LED heads with a lint free cloth and isopropyl alcohol (the LED heads are the attachments on the underside of the lid when you open your printer; four of them for each color). Try printing something after doing that. If it's still an issue just with black, try switching the black and magenta drums/toner (keeping them locked together). The drum w/ toner cartridge simply pulls straight up and out, no switches or locks to flip. Then print a page of text; which will output from the printer in magenta. If that looks okay, then it may be an issue with your black drum and needs to be replaced.

charging connections are not properly connected

It sounds like a defective drum unit. Also make sure you are using genuine oki supplies.