The problem is your FX 6100 and 750 Ti are both below the game's minimum required FX 6300 and GTX 660.
https://www.systemrequirementslab.com/cyri/requirements/assassin-s-creed-odyssey/17860
FX 6100 Passmark score = 5389
https://www.cpubenchmark.net/cpu.php?cpu=AMD+FX-6100+Six-Core&id=257
FX 6300 Passmark score = 6388
https://www.cpubenchmark.net/cpu.php?cpu=AMD+FX-6300+Six-Core&id=1781
750 Ti Passmark score = 3761
https://www.videocardbenchmark.net/gpu.php?gpu=GeForce+GTX+750+Ti&id=2815
GTX 660 Passmark score = 4130
https://www.videocardbenchmark.net/gpu.php?gpu=GeForce+GTX+660&id=2152
Meeting the GPU and CPU requirements is actually more important than RAM and VRAM reqs, though all are important and should not be ignored.
As for RAM, ALL PCs have to have it to run, so when you say you bought RAM, did you replace what was there with higher performance RAM, or add to it? Adding RAM has to be done right. For best performance using dual channel mode you need to look for the exact same frequency, capacity and timings (mostly CAS timing), and preferably the same brand/model. If not, the system will default to the slower module. Adding RAM improperly can also cause stability problems, and you need to set it to XMP for it to even run at it's rated speed.
Hard to say whether an OC on the CPU and GPU would solve the low spec problem. The GPU is within 10% of a 660, so you might be able to equal it's performance with a slight OC, but I'd use a higher fan curve to keep it cool.
The CPU though is a good 19% slower, so that will take a fairly robust OC and you definitely want a good aftermarket cooler for that.
It will all go smoother if you have a case with good ventilation. If you're not familiar with overclocking, you'll want to read some good guides on it to make sure it's done right.