Quantized error in scatter plots for a hamming windowed LTE signal
That's the picture you'd get usually if you had a single-carrier system with a slight IQ imbalance. (but that would usually be kind of slanted; but we don't know what your OFDM receiver does, so we can't tell you.)

In a multi-carrier system like OFDM, I'd look into whether I accidentally added inter-carrier-interference (ICI), for example by having an uncorrected fine frequency offset. Or, and that's not that uncommon for simulations, by having a control loop trying to achieve frequency synchronization, but goes wrong. (But that would usually be less "nice and orderly looking". However, we don't know what your OFDM receiver does, so we can't tell you.)

Since this is in simulation: Test. Send a stream of constant symbols, from only one subcarrier. That way, you can see where the effect comes from. Iterate. Debug. 

You're the engineer sitting in front of that problem – go and investigate!