In business terminology, opportunistic is usually placed in contrast to strategic (google the terms "opportunistic vs. strategic" and you'll get literally millions of articles from the business press).
Strategic business is deliberately planned in advance. Probably the marketing department has identified and advertised to some likely segment of potential customers; the sales department has actively prospected for leads in that segment; and sales orders have been placed by the people contacted through this strategy.
Opportunistic business is not strategically planned. Either it arrives completely unsolicited (maybe someone stumbles across the company website and places an order), or it comes through the sales channels in a some unplanned way (maybe a sales representative is at their child's school event and discovers that a fellow parent wants to buy what the company is selling).
Typically, strategic business success is much easier to repeat reliably than opportunistic business is, so identifying older sales orders as "opportunistic" is often given as an explanation (some might say an excuse!) as to why that success wasn't repeatable.
As for your second question, "a year over a year" sounds incorrect. You do very often hear "year-over-year" (no articles) in discussions of business results. For example:
Sales decreased in [XXX] year-over-year.
As Spitemaster said in their answer, this expression means "comparing or based on comparing the same time period in successive years." This phrase is used so often that most people who are used to seeing financial reporting will recognize the abbreviation YOY without needing to have it explained.