Without the context of the surrounding community -- knowing the friends, neighbors, and associates -- we can miss important clues. We can start by narrowing the time frame for when Samuel McReynolds came to Oregon, and then working backwards from there.
A search in the FamilySearch catalog for Umatilla County, Oregon shows:
The Fresno Bee newspaper article you posted in your other question said Samuel had been involved in mining, so it might be worth it to take a look at these Umatilla county mining claims:
Miners travel all over the country to find work. If you can find other people who came to the area from Missouri, that might give a hint as to friends, associates, and neighbors of Sam's family back home.
The other record sets I would check, just in case, are these:
Being born in December 1847, Sam is perhaps too young to have served at the beginning of the war, but he might have been called up at the end. A quick check for other men named Samuel McReynolds could also give you some hints about how many same-name candidates you'll have to contend with when you search for records in Missouri.
If you find candidates in the National Park Service indexes, sometimes a search of Google Books will turn up published rosters and histories of the units they served in.
Another untapped resource is to widen your search and study the area itself -- find bloggers who write about other people who lived in the areas you are interested in. You may not find your own family in their writings, but if they talk about how they found records and maps and other things, it will give you ideas of where else you can look for things. While I was searching for information about the town my father's family came from, I discovered a blog post with a railway schedule. I hadn't looked at the map, so I hadn't considered how people might have traveled on the train.