So I checked, and I really thought I used CSYNC OUT on the CXA1645, but it doesn't have a CSYNC out .. I mixed that up with the CXA1145.
I actually used Chroma to sync
Somehow the OSSC managed ot make it work, but that not supposed to be like that.
So, next try.
I could use a THS7374 as amp and a few passives to get a 75Ohm signal.
Or I can use a EL1883 sync stripper, this one is much faster than the LM1881.
I had one of these around and a tiny PCB for it that I always looked for an excuse to use:
oshpark.com
Isolating the CSYNC Pin on the PCB:
Quick&Dirty test setup:
Taking the TTL CSYNC from the Pin 10 of the CXA1645 as input, on the CSYNC output from the EL1883 there is a 470 Ohm resistor in series and then a 75 Ohm resistor against ground, this yield around 0.6V CSYNC which is fine for 75Ohm consumers, it you want 5V TTL, leave both resistors out.
By using TTL CYNC as input, I avoid delay caused by the mixer that creates Composite Video, also the ES1883 is faster than a LM1881 and won't cause delay/shifted picture.
It does not change that the OSSC needs the timings adjusted or it won't sync to it.
Edit:
it can be done with two resistors and a cap
from the CXA pin 10: a 10k resistor in series to a 470uF cap (negative leg to the AV port) and a 1.8k Ohm pull down results in 0.6V CSYNC
using lower value serial resistor will reduce the actual TTL CSYNC Vpp, not recommended