In the first sentence, "its" refers to OPEC, not its members. The sentence is saying that members of a group comply with its decisions. Here I'll use bold & italics to link the matching terms:
of the Organization of Petroleum Exporting Countries
This makes more sense than the alternate interpretation, that the members comply with their own individual decisions.
In the second sentence, however, "were" makes sense, as it is the members, and not Congress, that were surprised. Here, we can ignore the modifier "of Congress" and just say "Members were surprised."
I hope this helps!
Dmitry Farber | Manhattan GMAT Instructor | New York
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