Normally your training/type school should brief you thoroughly on what is expected in terms of the oral etc. Normally the oral is limited to:
1. Limitations
2. Memory Items.
3. Walk around slide show questions.
4. System questions usually conducted while reviewing the cockpit panel in the classroom.
All this can take around 45 mins to 1 hr. It all depends on the examiner and how you progress. If you get stuck along the way they start digging deeper. If you have never flown FAR 121 then they might not ask you much at all.
Although it would be good to know some of the rules, mainly pertaining to flight ops (alternare requirements, f/A requirements, etc etc).
good luck