An opinion is a belief which is not itself objectively factual. Factual information can certainly be a basis for them. If we go back the advertising example, we could take two objective facts about advertising:

* online advertising allows consumers to learn about new products and services

* advertising incentivizes user data collection in order that it may be more effective.

From just these two facts one could easily come to a pro- or anti-advertising position based on their values and the relative weights they choose to put on each fact.