As others have mentioned, flights from Ottawa to Orlando can vary a lot. In recent years (actually about 5 years ago), I think the cheapest I've flown from Ottawa was $550/person non-stop return (we always try to fly non-stop). I've seen $800-$1000+/person around XMas time.
Right now, according to flights.google.com, for flying from Saturday Aug 19 to Sat Aug 26:
The cheapest you can get YOW->MCO is $435 but it's with 2 layovers and 12 hours of travelling. A more reasonable option would be $502 with 1 stop (Montreal) and a 5 hour total travelling. Cheapest direct (both ways) flight is $685 and 3 hours of travelling.
If you fly from Syrucuse (SYR->MCO) (on a major airline), you can do $338 with 1 stop and 5 hours of total travelling. $499 direct and 2:40 hours of travelling.
If you fly from Ogdensburg and land in Sanford not Orlando (OGS->SFB), it would be about $300CND (direct) on AllegiantAir BUT they do not appear to fly mid-August to end-September (last flight would be Aug 5-12).
If you fly from Plattsburgh and land in Sanford (PBG->SFB) on Allegiant Air, it would be about $350CND (direct) but that's for Fri Aug 18 to Fri Aug 25 (no Saturday flights).
If you fly from Syracuse and land in St-Pete (SYR->PIE) on Allegiant Air, it would be about $400CND (direct) but that's for Sun Aug 20 to Sun Aug 27 (no Saturday flights).
If you fly from Montreal (YUL->MCO), it would be about $506CND (direct).
If you fly from Toronto (YUL->MCO), it would be about $506CND (direct).
The above prices do not include luggage fees but I think they are all fairly similar expect that AllegiantAir does charge for carry-on as well as checked luggage. And AllegiantAir does charge from pretty much anything they can (seat selection, no free food (although I don't know if any airlines give that on economy flights either), priority boarding, etc but we never get any of those anyway). The other small consideration is parking costs. I believe it's cheaper at pretty much all US airports but once you factor in the exchange, it might be close. Personally, I'd rather cross the border by car instead of going trough customs at the airport so that's another benefit for us of not flying from YOW, YUL or YYZ. I also prefer driving to a US airport than driving to YUL or YYZ. The last few times we flew from Plattsburg, I think we got a hotel and 2 weeks of parking and shuttle to the airport for about $200CND.
Since 2010, we pretty much only fly with Allegiant simply because it's the cheapest for us especially since we really want direct flights. As a family, since then, we've flown 18 times with them and my wife has flown 2 other times with her mom. During that time, we've had a few delays (the longest was about 4 hours) but no cancellations and most of the flights were on time or within 15-30 minutes (which I think is pretty typical) (If you look at flights.google.com for YUL->MCO, it has a warning that 44% of the time, the flight is delayed by 30+ minutes). That said, flying if you fly with AllegiantAir, there is no flexibility in the dates and if the flight is cancelled or if you miss your flight, you're kind of screwed.
If you are interested in flying from Ogdensburg at the end of August, I would strongly recommend that you write to them suggesting they add flights (which they do when there's enough demand). Travel to Flolrida typically reduces quite a bit mid-August because most kids are back in school in the US and I suspect they based their OGS schedule on that. I've seen other people asking the same thing so it doesn't look like you are alone.
Hope this helps,
Mike
) with Delta with layovers both ways. If you are willing to drive a little more, the best deal we got was direct flights with Southwest from Manchester (NH), which was 1380$ US all included (well, we chose to not pay for Early bird so this would have been extra). This airport is 4 hours from home but it was worth it considering we usually drive 2 days to get there... 
