To be frank, you lose the fact that there's an "O" in #5, great logo, just that it doesn't convey the fact that it's "Ohio Western". #3 or #4 are both good.
I used the Norfolk Southern "NS" as an inspiration for #3. No. 5 would be on the nose of the loco, if I use it. I took the Southwest Airlines and London Underground logos as inspiration for #5. I'm leaning more towards #3 for the nose and maybe #2 or #4 for the sides of the loco.
I like them all, but indeed #5 doesn't look like it has the 'O'. What you could do is have one of the heralds (like #5), be the one for the engines, while have another one be for rolling stock, if you plan to have any cars in Ohio Northern.
I like #4 the best, then #1 or #2. I'm assuming that this is a fairly modern line, say mid-'80s or later? #5 is good, but looks more suited to an airline to me, and there's the issue of the indistinct "O".