I finally ran the strength of schedule numbers using both the OHL's points system (RW/OTW/SOW = 2 pts, OTL/SOL = 1 pt, RL = 0 pts) and the HockeyDB points system (RW = 5 pts, OTW = 4 pts, SOW = 3 pts, SOL = 2 pts, OTL = 1 pt, RL = 0 pts). No big surprises here: Guelph and their 28 games against the West's Big 5 had by far the hardest schedule in the league. (Guelph's average opponent finished with a better points total than the Mississauga Steelheads.) Despite having the same common opponents as Guelph, Kitchener's SOS is actually a fair bit lower than Guelph's because the teams played each other eight times (and Kitchener was much stronger than Guelph this year). Mississauga and their 34 games against the East's bottom six comfortably had the league's easiest schedule.
The West had the top seven teams in terms of SOS, with only Erie, OS, and SSM coming in below average thanks to their conference crossover games and being strong teams themselves. SSM had the third easiest schedule in the league. (Note how much stronger Windsor's schedule was than SSM's--the difference is two extra games against each of Erie, London, and OS vs. four extra games against Sudbury and two extra games against North Bay.) London's schedule was also a fair bit stronger than both OS and SSM's--I don't think it's unfair to argue that scheduling cost the Knights the third seed in the playoffs. Ottawa snuck in an above-average SOS by virtue of playing 24 games against Kingston, Oshawa, and Peterborough, three of the top four teams in the East. OMG67 has argued in the Peterborough topic that the Petes faced an light post-deadline schedule--the numbers certainly bear him out, as they had the fifth easiest schedule in the league.
Note that these numbers are biased--I did not remove the teams' head-to-head results in assigning their winning percentages for the purpose of calculating their SOS. (So, for example, in calculating Kitchener's SOS, the opponent's winning percentage I used for their eight games against Owen Sound included OS's 7-1 head-to-head record against the Rangers.) In a perfect world, we'd have unbiased numbers, but that would be much more difficult to compute (at the very least, we'd need more accessible head-to-head data), and the effect is likely minimal. But for the curious, unbiased numbers would typically move the stronger teams up slightly (they cause their opponents' records to fall) and the weaker teams down slightly (they cause their opponents' records to rise)--so you might want to adjust the numbers in your head accordingly.
TEAM ------------ OHL -- HDB
Guelph ---------- .602 -- .553
Saginaw -------- .581 -- .530
Sarnia ---------- .580 -- .531
Kitchener ------- .578 -- .530
Flint ------------ .570 -- .518
London --------- .566 -- .515
Windsor -------- .564 -- .514
Ottawa ----------.555 -- .502
Erie ------------- .549 -- .499
Barrie ---------- .545 -- .494
Kingston ------- .542 -- .489
Niagara -------- .540 -- .487
Owen Sound --- .539 -- .489
Hamilton ------- .535 -- .482
Oshawa -------- .534 -- .481
Peterborough -- .534 -- .482
North Bay ------ .534 -- .481
Sault Ste. Marie .530 -- .479
Sudbury -------- .529 -- .476
Mississauga ---- .520 -- .468