Because it is based on light, which has a large capacity to carry information, OCT has very high resolution.  It is 20-70 times higher resolution than ultrasound, and 200-1000 times higher than X-ray.

Additional general information:

Dual-Source Swept-Source Optical Coherence Tomography Reconstructed on Integrated Spectrum

OCT has much higher resolution than X-ray or ultrasound.

OCT covers a range of biological detail.