Research papers put facts before opinions. The body of knowledge is more valued than the author's ideas. The author is adding to the collective body of knowledge by the way he organizes information, what is included and perhaps some new, factual data. Any claim made in the paper should be verifiable.
Research papers use a formal style. Verifying facts in research papers is a differentiating factor. The way facts are to be verified in research is prescribed in writing manuals. The MLA style guide published by the Modern Language Association is among the most common. Research style creates a common thread throughout the entire body of formal research.
In opinion papers the logic of an argument is among a broader set of tools that can be used to move the audience. Anecdotes, humor, sentiment, endorsement and other approaches can augment a factual approach in assembling a cohesive and persuasive piece. This often results in editorial digressions where the author may state a fact informally, then share his opinion regarding the fact.
Point of view is a noteworthy difference in these styles of writing. In research papers, third person is the proper point of view. While opinion papers have relatively few restrictions, they share a restriction here: they use first person point of view. They may not rely heavily on first person narration, but they don't step into other POVs. To do so would confuse the reader because the choice of POV helps tell the reader whether the information they're consuming is fact or opinion.