OneSearch searches multiple databases at once, which can save you lots of time by not having to individually search through multiple databases.
Best reasons to search individual databases:
1) When the assignment requires specific databases (ex. Genderwatch)
2) You have not focused on your topic (I recommend an "in Context" database or database to look at the topic overview for narrower topics)
3) The information is not in OneSearch (ex. statistics or literature criticism)
Depending on your topic, you might want to use a subject-specific database. Genderwatch covers several topics from feminist theory to gender identity. Use the following tips to locate search subject-specific databases. Not sure which database to use for your topic, read the description or ASK US!