What is this subfloor, and do I need a wood underlayment for hardwood
Yes, that's OSB. It's the only product that's made with large wood "strands". I wouldn't have guessed that it was 3/4" though due to the apparent lack of tongue and groove along the longitudinal edge, but maybe the photo is misleading.  Particle board looks like compacted pine sawdust. Fiberboard (MDF) looks like pressed pulverized wheat chaff. Neither of those are used as subfloor.
No, not all wood flooring requires underlayment. Modern t&amp;g subfloor in the 3/4" class is adequate for some products. However, a second layer can result in a stouter, quieter finished floor which feels higher quality. Read the label.