Most OMAS fountain pens are piston-fill only, meaning you will use bottled ink to refill your OMAS fountain pen. A few models refill with standard international-sized ink cartridges.If your OMAS fountain pen is piston-fill only, choose a bottle or two if ink from our large selection. Colorado Pen ink arrives in a huge 90ml bottle. Pelikan offers standard ink or its premium Edelstein ink in nine vibrant colors. Waterman bottled ink is offered in all the favorite colors.If your OMAS fountain pen fits ink cartridges, Colorado Pen offers international-sized ink cartridges in 12 different colors. This specially formulated ink helps keep your OMAS fountain pen writing smoothly.If you have an OMAS cartridge fill fountain pen and have the ink converter that came with your pen, you can opt to economically refill with bottled ink. Again, look carefully at the huge selection of bottled inks that can be loaded using your OMAS ink converter.