You need to have your medical completed before USEM interview, It takes approximately 1 week for the medical results to be delivered to USEM, so schedule accordingly.
You book the OFC (bio) first, then the USEM interview after, this is done all at the same login. You can't book OFC only, if you do that the system will cancel the appt.
So what I did yesterday was book the OFC/SLEC two consecutive days, then a month later the USEM. My Pinay sweetheart lives in Mindanao so this means two separate airplane trips to Manila, but thats just the way it is cant do anything about it.
Hope that answers your questions.