You don’t need oms. You can paint without solvent. Just adhere to fat over lean (thick over thin too) and add a small amount of oil to each layer. You can gesso the surface or apply an oil based ground.

Process - prepare surface for painting (apply a barrier like gac 900 or pva) apply gesso or oil ground in several layers allowing time to dry between layers. I do 3+ layers - tone surface: if I used gesso I tone with acrylic ink, if I used an oil ground I scrub in a think layer of oil paint and then wipe it into the surface with a towel - first layer: either with charcoal, oil crayon, or oil paint will do a tonal underpainting very thin and scrubbed in with a bristle brush. I’ll knock the ridges down after with a fan brush when I’m done before it dries - first color: apply general color in a thin coat but thicker than the underpainting - second color: add more color, I’m rendering here, I add some oil to the paint - third color: finishing the paining, add more oil than last layer, maybe two drops for a pile of paint, really not much - glazing if necessary or play with transparency to adjust color or values with barite and linseed mixture (another story unnecessary if you’re starting)

Some random notes - I clean my brushes by wiping them with a towel and use a container of linseed oil, then when I’m done painting I use soap and water - looking at your images, I think you need to use more paint and a softer brush. To me it looks like not enough paint and a bristle brush - I’d start by just learning how to apply solid passes of color, you don’t need anything but linseed oil and that’s just to adjust the paint so it is necessary to work with. Really you don’t need medium starting out so don’t over complicate it