The answer to this question depends largely on what you mean by the “early 1800s.” There were two main motivations for American movement into the Oregon Country. Some of this movement happened in the first decade or two of the 18th century while the next wave began late in the 1830s and then really boomed in the 1840s. Either of these waves of movement could be dated to the “early 1800s.”
The first wave of American movement into Oregon was very small. This was a movement that was caused by the desire to participate in the fur trade. By 1811, for example, the Pacific Fur Company had started to build Fort Astoria as a trading post at which it could obtain furs. Americans who went to Oregon during this time were largely interested in participating in this trade.
In the late 1830s and early 1840s, many more Americans started to move to Oregon. This was the era of the famous Oregon Trail. These settlers were motivated by a desire for land, not by a desire for furs. They were not going to Oregon to trade but rather to set up their own new homes. They had heard that Oregon (in particular the Willamette Valley) was very fertile and had good weather. They wanted to set up farms in that area, thus giving themselves the chance to become wealthier and more independent than they had been farther east.
Thus, the first (smaller) group of Americans came to Oregon for furs while the second (bigger) group came to settle on the lush farmland of region.