Well, OPEC does collude. That's why it was formed, and its members make no bones about it. Members get together in Vienna and agree how much oil each will supply to the market.
It is absurd to assert that OPEC members can be subject to US antitrust laws. Its members are sovereign nations, not companies doing business in the US. Unlike the US, the oil in OPEC member states is owned by the government, not private oil companies.
Maybe, but I was specifically referring to American oil/gas companies.