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"If Kim Jong Un takes that approach, we'll be extraordinarily disappointed and we'll express that disappointment," O'Brien said on ABC's "This Week" when asked whether the US would respond to a long-range test.
American officials say they don't believe they are out of the woods yet and are closely monitoring events on the Korean Peninsula, according to a senior administration official.
That includes monitoring a meeting of top-level ruling party officials, which North Korean state media said had convened on Saturday. The Korean Central News Agency said the plenary session was meant to address the "manifold and harsh trials and difficulties" facing the country, and would continue into another day.
It's possible North Korea could announce changes of policy direction at the meeting, including an intention to end diplomatic efforts with the United States or restarting its missile or nuclear testing.
In the past, Kim has used the plenary sessions to announce important foreign policy shifts. He is due to deliver a speech on New Year's Day, where any decision could be announced.
O'Brien said Sunday that the White House was monitoring the situation, which he indicated was concerning. But he declined to speculate on specific consequences for North Korea, saying only that the US has "a lot of tools in our toolkit."
It is not known, however, what the threshold of North Korean action would be to generate a US response. North Korea has fired several short-range missiles in recent months without any US military response.
"The President and Ambassador Bolton didn't always see eye to eye on how to conduct American diplomacy," O'Brien said, adding that Trump doesn't have any "illusions" about the difficulty of opening talks with Kim.
Asked whether the US and North Korea had communicated since October talks broke down with the US special envoy Stephen Biegun, O'Brien said only that "there are channels of communication."
CNN's Barbara Starr contributed to this report.