Hi,
I just had my OFII appointment last week so I can give you my account. Mine was in Montrouge (dept. 78). FYI I understand spoken French pretty well and can have very basic conversations. Here's what happened:
---I showed up at 1:00 for my appointment time, along with about 20 other people who had the same appointment time. We were let in the building one by one after a bag check by a guard. The lobby secretary just said, "Passport" to me, which I gave her, along with my paper showing I had an appointment (take that with you!). She pointed upstairs told me to go to the second floor.
---2nd floor: small conference room where a nice OFII worker came in and started telling all of us in French how we were all in that room to watch a "welcome to France" type of video. She then explained how the rest of the half day would go, including the medical test and the certificates we'd get at the end of the half day. All of this was in French. At some point she asked (in French), "Does everyone here understand and speak French?" and everyone chimed in "Oui," and I then realized I was the only non-native speaker there. Everyone else was from a French-speaking African country and/or was otherwise perfectly fluent. I think they had somehow assumed I am more fluent than I am, because I know they put some (completely) non French speakers in a different room. Even if I spoke no French, I could have sat there silently and faked it until they called my name.
---The video lasted about 15 minutes and was stuff like "here in France, husbands cannot tell their wives not to work," etc. etc.
---Next, in seemingly random order, we were called individually by name. This took the longest out of any part of the day. I was called second to last so I probably spent an hour waiting to be called.
---When called, they sent me to a little room for height and weight measurements. No need to speak French there, it's pretty obvious to stand on the scale, etc.
---Next, into a little office to chat with an employee who asked me to verify on her computer screen my name, birthdate, etc. This was the time when I think they "unofficially" tested my French, because she didn't speak a lot of English. She asked questions like if I work, where I work, how long I've been here, etc. *She told me about the language classes (which I said I didn't need and she didn't really push it because she could see that I understood her well enough to reply...I think the classes are really for people who speak NO French?)...she also asked me if I needed to see a social worker. I said I had no need. Finally, she told me about the mandatory half day of civics class that every person who comes through the OFII has to take, regardless of French proficiency or how long you've been here. She offered me two locations close to where I live and two possible dates. They only offer two dates and if you can't go to either, it's a hindrance to renewing your visa the next year, etc. So make sure you can go. That was it for her part.
---Next, the chest xray. Men and women both go naked from the waist up into the xray room and it takes two minutes max. (*Note for ladies reading this: Some women say they had to walk down a hall half naked. This location had a little "changing room" and the only person who saw my boobs (ha) was the technician who did the xray, and he sees 54354230322 boobs a day, so who cares.
---Then, wait 10 minutes or so for another nurse (?) to call me into her little office. She didn't speak English, it was all in French. She listened to my lungs with a stethoscope and then showed me my xray film (that was quick!) and told me I had no problems (no kidding, sheesh). She asked me the standard list of questions a doctor might ask about medical history: have you had surgeries, cancer, operations, etc. It was all yes or no. ***She asked me if I take any medications and I lied and said no (ha) because I do but nothing serious or worth mentioning. She asked me if I had any vaccination records from the U.S., and I said no, which is true, so she asked when is the last time I was vaccinated for something or other (I can't remember), and I told her I can't remember.

She said, okay, "you're supposed to get vaccinated at the age of (blah blah) and/or (blah blah), she handed me a little card with the ages and info, and that was it. So in summary, you don't really have to share your medical data unless you have some pertinent issues to share.
---Someone else at a desk also asked me if I had medical insurance and through who. That part was a little complicated for me to explain in my sketchy French, but she was patient and finally got that I'm covered under my husband's plan. (I have my own carte vitale.)
---Finally, I paid the ridiculously high fee in stamps, they gave me a receipt. They gave me my carte de sejour, which is like a little driver's license. Then I was free. So it took a half day, like they said.
So overall, I would say that having some French knowledge was helpful, but I know they see people all the time who don't understand or speak French and they help you through it. If you have to go by yourself, don't stress about it.
Good luck!