If it got chilly at night or in September on the trail, woolen clothing came in handy. It would have been too heavy to wear during the heat of the day.
It was a long process to get from the sheep to the shirt.
I found some really good pictures. They are from Wikipedia Commons except for one which is from USDA and I will label it.

First are the two maps.

Oregon Trail  by, National Park Service

NASA topographical map with trail line by, Matthew Trump.

I’ll start at the beginning of the process.

By, Dickbach

Next are two pictures of wool.

By, Cgoodwin

By, Ken Hammond, USDA

This is a picture of spinning wool into yarn.

By, Jeekc

Two pictures of colorful wool yarn.

By, sahradesertfox

By, Roy & Danielle

A picture of a wool sweater.

By Liralen Li

The last picture is of a ball of wool yarn with a cat attachment  by, Loliloli.