St. Louis Cardinals closer Seung Hwan Oh was “completely unhittable” for the majority of his MLB debut season. With this in mind, over the offseason, I wrote (using a GIF from the great @cardinalsgifs) about why Oh was so difficult to hit in 2016. Well, so far in 2017, the Final Boss (or the Stone Buddha, if that’s your preferred Oh nickname) has provided the Cardinals with replacement level value (0.0 fWAR through 23.0 IP) out of the bullpen, largely because his strikeout rate is down significantly to 19.2% (from 32.9% in 2016) and his walk rate is up to 9.6% (from 5.8%). Of course, Oh struck out four of the five Dodgers he faced in his most recent outing on Tuesday, but that certainly has not been the norm this season. What exactly has changed, you ask? Let’s take a closer look.

Oh’s overall pitch usage thus far in 2017 is about the same as it was in 2016. The fourseamer remains his most frequented pitch at 58.48%, though it is down roughly two percentage points from its 2016 usage rate (and this slight difference has been replaced by more changeups). Either way, the change in usage rate is minor, so there is nothing really noticeable in this aspect of Oh’s approach to hitters. Thus, for just about any fastball-dependent pitcher, the typical first step is to look at velocity. I’ve heard rumblings of a velocity decline for Oh, but honestly, this really hasn’t been the case as his fourseamer is averaging 93.41 MPH this season, as compared to 93.53 MPH last season. Since there is more to a pitch than velocity, the pitch’s movement profile is virtually identical as well.

While the fourseamer velocity may be the same, there is what I believe to be an important distinction to make regarding Oh’s average changeup velocity. He has experienced an increase in changeup velocity (to 85.00 MPH from 83.57 MPH in 2016). The shortening of the gap between the respective velocities of the changeup and fourseamer can play a vital role in hampering the pitch’s success. Sure, the pitches’ averages still meet the desired 8-10 MPH difference threshold, but for a pitcher who isn’t blowing too many hitters away and relies primarily on deception, even the smallest change (no pun intended) can play an influential role.

Given the title of this post, let’s see how Oh’s 2017 whiffs per swing rates compare to those of the 2016 season:

Seung Hwan Oh, pitch-by-pitch whiffs per swing Pitch Type 2016 2017 Pitch Type 2016 2017 Fourseamer 29.04% 26.67% Changeup 42.22% 38.10% Slider 45.19% 28.33%

Just as you’d expect from a pitcher who is experiencing a drastic decline in strikeout rate, Oh’s whiffs per swing rates are down for each one of his pitches (yes, Oh also throws a curveball, but it’s so rarely used that I didn’t feel the need to include it in the table). The significant difference between the slider whiff rates is alarming. While “Oh rode his fourseamer to early MLB success” last season, the slider quickly became his go-to put-away pitch. Here is how hitters fared against the pitch last season (and for comparative purposes, I’ll include the pitch’s 2017 results as well):

Results versus Seung Hwan Oh’s slider Year AB BAA SLG ISO Year AB BAA SLG ISO 2016 110 .164 .246 .082 2017 36 .333 .667 .333

Remember, because they only take into account the final pitch of a plate appearance (and thus diminishing the importance of sequencing), I don’t put too much stock in individual pitch results. However, when the differences are this drastic (and more balls are being put in play), it’s definitely worth noting. Thus, not only is Oh’s slider inducing fewer whiffs thus far in 2017, its results upon being hit have not been desirable. This is further confirmed by the yearly launch angle charts (made available on BaseballSavant.com), as the slider was inducing considerably more ground balls (anything below a 10 degree launch angle) last season. In fact, the average launch angle for 2017 is of the line drive variety at 14.3°.

So, what has been different with Oh’s slider? Other than being thrown slightly softer (85.98 MPH versus 86.29 MPH), the pitch is experiencing a smaller magnitude of dragless horizontal movement this season (vertical movement is similar). In 2016, the pitch averaged 3.38 inches of glove-side horizontal movement. Thus far in 2017, its average is only 2.07 inches — which is counter-intuitive considering it’s being thrown at a slower velocity. Again, while the difference in movement may seem slight, it’s of major importance for a deceptive pitcher like Oh.

When a particular pitch is experiencing less movement, location becomes even more important. In 2016, Oh was consistently enticing hitters to chase out of the zone. So far in 2017, that isn’t happening, and he has been forced to bring the pitch into the strike zone. Given its limited movement profile, this has left the pitch susceptible to being hit hard by opponents (as already referenced above).

Bottom line, I understand that Oh’s success depends on more than just his slider, but at the same time, it’s a pitch he simply must get back on track if he wants to become a force out of the bullpen again. Finally, let’s close out this post with one of @cardinalsgifs trail-blazing GIFs. Slider number one took place on June 3rd, 2016, against Brandon Belt and resulted in a whiff. Slider number two took place on May 1st, 2017, against Travis Shaw and resulted in an absolutely crushed game-winning home run.

While the slider to Shaw may look like it possesses more horizontal movement than the one to Belt, take note of the wider release point (which seems to be trend in 2017) and the mitt of Yadier Molina on both pitches. When you look into the PitchF/x data itself, the 2017 slider to Shaw actually backed up (meaning it tailed toward the end), Plus, if you follow Molina’s glove on both pitches, it’s easy to tell how much sharper the 2016 slider was. Here’s to hoping Oh can bring back his 2016 slider going forward.

Credit to BrooksBaseball.net, BaseballSavant.com, and the great @cardinalsgifs for providing contributions to this post.