Attack dog is an unofficial role in political parties:

An “attack dog” in politics is a person, usually a politician or a political operative, who is designated to aggressively challenge and criticize the opposition.

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They are often tasked with the responsibility of delivering negative messages or criticisms that a candidate or party leader may not want to deliver personally, in order to maintain a more positive public image.

He has done this in the past and been effective at it. His successful (unjustified) criticism of the Liberals carbon tax implementation is a great example of that: he managed to frame public perception of the rebate, despite heaps of evidence to the contrary.

Shitting on Poilievre is fine and fun, but it ignores the reality that his party has successfully framed a number of debates in the past. Assuming Carney wins the Liberal leadership, it's a significant obstacle to him winning the next federal election.

The official opposition is a part of the government.

From the House of Commons website:

• The leader of the party having the support of the majority of the Members of the House of Commons is asked by the Governor General to form a government and becomes the Prime Minister;
• The party, or parties, opposed to the government is called the opposition (the largest of these parties is referred to as the “official” opposition);