Opposition is an important consolation for an electoral runner-up in Canada's Parliamentary system. Official status brings both prestige and profile as a 'government in waiting', and the very real benefits of money and resources for administration and research. Compared to conventions for determining the winner though, there are only a few practices in place to choose who is entitled to take the residence at Stornoway, the official home in Ottawa of the Leader of 'Her Majesty's Loyal Opposition'.
Based on the limited precedents, and the uncertainty of outcomes in the current 3-way election, there are a few possible scenarios in the event of a second place tie.
The governing Conservatives win and Liberal and New Democrats tie for second place in the number of seats.
Unlike concerns about who would form government, determined by the Governor-General, Opposition status is determined by the Speaker of the House of Commons. The key factor in making such a decision in the event of a tie would likely be incumbency.
That is, the party that served in the Opposition role in the previous Session of Parliament would likely be awarded Official Opposition status if the second place parties had the same number of seats. In this hypothetical, this would mean the current Opposition, led by Thomas Mulcair's New Democrats, would retain their official status
Current Opposition, the New Democratic Party, wins election and the Conservatives and Liberals tie for second.
In this case the incumbency principle would not apply. Though there appears to be little precedent, I would argue that a reasonable corollary to the incumbency principle would be to award Official Opposition status to the party with the higher standing in the last Parliament. In this hypothetical, as the former government, the Conservatives would consequently win the right to occupy Stornoway.
Justin Trudeau’s Liberals win and Conservatives and New Democrats tie for second in the number of seats.
What would happen in this situation seems uncertain. As the incumbent Opposition, New Democrats would likely argue that they should form Opposition. However, the Conservatives might well have an argument that, as the defeated government, their status in the previous session meant that the incumbency principle should be superseded.
In this case, few precedents and no clear rules might mean other considerations are taken into account. For example, the Speaker might well apply other minor factors to help determine the matter, such as who amongst the second place parties had the higher share of the popular vote.
To be fair, all 3 scenarios described above appear improbable. However, it’s worth remembering that unusual things do sometimes occur in Canadian politics. Who would have predicted in advance, for example, the unexpected and seemingly unprecedented request by Prime Minister Harper for prorogation in 2008, in the face of the prospect that he would be defeated in the House of Commons within a few weeks of the previous election?
In a similar unlikely, but not impossible circumstance, that there was a tie for second place in the 2015 election, who would form the Official Opposition and be entitled to the keys to Stornoway, is not entirely clear.
 Usually the party that wins the second highest number of seats in Parliament. For a good overview see Stewart Hyson, “Determining the Official Opposition in New Brunswick and the House of Commons”, (1996) Canadian Parliamentary Review, Vol 19 No 3. A
 Though not exactly factually consistent with the scenarios set out below since ties occurred during the legislative session, rather than immediately following an election, the two modern precedents occurred in New Brunswick in 1994, see Speaker’s Ruling “Tie or Equal Number of Members in Two Opposition Parties” Journals of the Legislative Assembly of the Province of New Brunswick, Third Session of the Fifty-second Legislative Assembly, December 16, 1994, pp 330-335, and; briefly at the Federal level in 1993, ibid.
 Supra note 1.
 In 1983 the Alberta NDP was granted opposition status in part on the basis of its popular vote. Another possible factor that might apply immediately following an election might include party status, if there was an organized party as compared to a number of independents