When looking at your search results in OneSearch, you'll see some filters alongside the results. One of them is called Scholarly Journals. Click that filter and the results will be limited to articles from scholarly journals, most of which are peer-reviewed.

Another strategy is to go directly to a database that is mostly made up of mostly of scholarly journals. Two big ones are Academic Search Complete and JSTOR. You can find more using the descriptions on the Research Databases page.