Very. The Orioles had the 4th highest OPS differential ever (from 1914-2014). It was .136. The Mets had just .017 (and several NL teams had better that year). So the Orioles had an advantage of .119.
The only time there was a greater difference before 1969 for the teams meeting in the World Series was in 1944 when the Cardinals had an OPS differential of .130 and the Browns had just .005. So the Cards had an advantage of .125. But that was a war year.
The 1927 Yankees had the greatest OPS differential of .196 and their opponent, the Pirates, had .084 (which is very good). So the Yanks had an advantage of .112.
Thru 1969, these are the only cases of an advantage being at least .100. There were also only three other cases reaching even .080. 42 cases had an advantage of less than .050. And that is out of 56 World Series.
So the Orioles came into the 1969 Series with an incredible edge over the Mets but the Mets still won.
If we go by OPS differential, 1969 was the greatest upset ever. The next biggest was
1931: A's had a .109 differential, Cardinals .045. A's had a .064 advantage yet Cards won.
So with an advantage of .119, the Orioles losing to the 1969 Mets was
not just the biggest upset (at least since 1914), it was far and away
One year before 1914 that was a big upset was 1906 when the White Sox beat the Cubs. But we don't have opponents SLG for before those years. But we can make an educated guess about how big of an advantage the Cubs had.
The Cubs had an OPS of .667 on offense while the Sox had .588. That gives the Cubs an advantage of .079.
The pitchers of each team allowed an OBP of .280. For the Cubs to have an edge of .119 in OPS differential, they would have to have allowed an SLG of .040 less than the Sox.
The Cubs did allow their opponents a batting average of .207 while it was .239 for the Sox. Now if each allowed the same isolated power, then the Cubs would have an SLG which is .032 lower. That would bring their advantage up to .111 (.079 + .032). That would make 1906 the 4th biggest upset.
The Cubs allowed 12 HRs and the White Sox 11. That suggests that they had the same isolated power allowed. But if the Cubs had allowed an ISO that was .008 less than the Sox, it would bump their advantage up to .119 and the upset would be just as big as the one in 1969 (SLG = AVG + ISO).
Since ISO is extra bases divided by ABs, it would mean that the Cubs would have to have allowed about 39 fewer extra bases (based on 4,918 ABs, which we can get by dividing their hits allowed by .207).
That would mean something like the Cubs allowing 20 fewer 2Bs and 10 fewer 3Bs. That would be 40 extra bases. That seams plausible. But we don't know for sure. There is just a possibility that 1906 was a bigger upset than 1969.