You should provide clearer resolution criteria

The Official Opposition has always been the 2nd largest party in the House of Commons, and is formally recognised in Erskine May: Parliamentary Practice. See section 2 of this note cited by wikipedia:

The importance of the Opposition in the system of parliamentary government has long received practical recognition in the procedure of Parliament. In 1937 statutory recognition was accorded through the grant of a salary to the Leader of the Opposition. The prevalence (on the whole) of the two-party system has usually obviated any uncertainty as to which party has the right to be called the ‘Official Opposition’: it is the largest minority party which is prepared, in the event of the resignation of the government, to assume office [and in a footnote: The Speaker’s decision on the identity of the Leader of the Opposition is final (Ministerial and other Salaries Act 1975 (c27) s2 (2))].

(emphasis mine)

What edge cases do you have in mind?

I suppose it's possible the two largest parties form a coalition and it's a smaller party left being official opposition, or that there two parties with an equal number of seats and equal claim to official opposition. But the odds of either of those happening are negligible.

It could be that the party with second most seats did not get the second most votes for example.

I'm not @LukeShadwell, but:

Formally, the Official Opposition is whomever the Speaker chooses.

Practically, the Official Opposition is whichever non-governing party (party not in the governing coalition) has the most seats. Vote percentage is irrelevant.

• @mongo If the 2 largest non-governing parties have the same number of seats — that's an actual edge case.

  • Likely they will negotiate — perhaps they may agree to be Official Opposition on alternating years (Parliamentary sessions).

  • But if negotiations fail, then it'll be the Speaker's decision. This will probably cause a minor constitutional crisis.

Yes as @puffymist says, the official opposition is well defined, there are no other criteria

If the second and third parties have equal seats I will cancel the whole market.