Could an Ogryn use a regular two-handed weapon as a one handed weapon? For example, a chain axe. Besides, how do you know which melee weapons are one handed or two handed? There is no such category in the weapon list.
Rogue Trader itself says nothing about any of your questions. Ogryns aren't even mentioned at all there. So I checked Only War.
Only War doesn't say anything about ogryns being able to use two-handed weapons one-handed, but I think it would be reasonable for the GM to allow on a case-by-case basis, given that ogryns start with Auto-Stabilized, Size (Hulking), and Unnatural Strength.
The individual weapon descriptions in Only War state "This is a one (or two) handed weapon.", but, for some reason, handedness is still not included in the weapon stats chart.