There are two major things you could do. You go back and spread fast drying plaster over the plaster cloth, and as it starts to set up, you could take a whisk broom and brush in a horizontal or diagonal texture. Then you get some earth town and dark green acylic paints and paint the plaster after it dries. These are sold in tubes in cradt stores. You use a about 1/2 round brush and and just dab on ground cover colers in random fashion. I us burnt umber, raw umber, burnt sienna, raw siena and Hooker's green. Then for black or grey I use Paynes Grey.
A second approach wold be to paint the plaster cloth as it is again using some sort earth color but not so detail as above. Then go to the local hobby shop and get some woodland "coarse" ground foam in earth and green colors. after the paint is dry, you spray on a spray adhesive, mix white color and water and paint it on, then sprinkle the ground foam over the area to you liking. I use old spice shakers from my wife for this. The woodland scenic shakers seem too big handle. Now if the texture of the clothe is too much, you may need to add more plaster. I would do that anyway.
I would do the ground foam on the first way also, but probably not so much.
Do not bother with "fine" textures, they are too powdery for O scale. Get a medium green, dark green, earth, soil and light green or burnt grass. The soil could be fine texture They also make "clumps" and "Foilage" which are much larger pieces you can use for bushes.
When doing scenery you can't screw up too badly. If you don't like the way it looks, just add some more different colors or textures.
You never did mention what type if hillside this is, just rocks, or more eastern grassy or wooded slopes.
