FL Studio’s automatic plug-in delay compensation (PDC) is notoriously bad when it comes to VST-instruments with multiple outputs (such as A4/Keys/Rytm-OB) and some additional steps are required to get OB plugins to sync properly with FL Studio. As these steps are certainly not obvious to everyone I thought it’d be great to spill the beans just in case there’s someone desperately looking for info on how to do it. 
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[li]Firstly, it is essential to route the A4/Keys/Rytm-OB plug-in to the MASTER track. That’s right; by not routing the OB-plugin output to any other mixer track than the master the automatic PDC won’t mess things up.[/li]
[li]Next, in order to properly route the individual outputs, from the OB-plugin wrapper properties go to ‘VST wrapper settings’ > ‘Processing’. From the ‘Connections’-panel the individual outputs can now be routed to separate mixer tracks as desired.[/li]
[li]Next, create a sub-mix from all the OB mixer tracks. In other words, route all the mixer tracks receiving audio from the OB plugin(s) to a separate mixer track (e.g. “Rytm OB”). Don’t route any of the OB-tracks to the master.[/li]
[li]Finally, a Fruity Limiter trick is used for syncing the OB tracks with the DAW correctly: Firstly, (1) place a Fruity Limiter plug-in on the OB sub-mix track, (2) turn the mix level knob in the mixer all the way down to 0% and (3) in the “Envelope” section of the Fruity Limiter adjust the ATT, REL and AHEAD knobs to 0%. Then, (4) find out the exact latency of the OB plugin – this is indicated in the OB-plugin wrapper settings (for example, my Rytm OB plug-in latency is 1915 samples, or 1915/44.1=43.42 ms (at 44.1 kHz sample rate)). The final step now is to (5) adjust the Fruity Limiter ATT knob (in the ‘Envelope’-section) to the same value as the OB latency (in milliseconds, e.g. 43.42 ms). In other words, the ATT-knob can now be used for manual latency compensation of the OB-plugin(s).[/li]
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This allows me to get all the individual OB outputs from my A4 and Rytm plug-ins in sync with the DAW. Most importantly, this also seems to do the trick even if I use plug-ins which introduce considerable latency in any of the individual OB mixer tracks (i.e. the FL Studio’s automatic PDC is working correctly). Just remember to route all the other stuff you’re sequencing in the DAW to a mixer track (instead of directly to master track) in order to sync everything correctly (this also applies to the metronome).