I am looking at a '68 Delmont 88 convertible. The top motor runs but the top doesn't move. Do these cars have a mechanical or hydraulic system? How hard/expensive are they to fix? And lastly, is there a way to bypass the system and manually put the top up and down?

I assume I would need a Fisher body manual before I tackle this project.

 

It will have a hydraulically operated top. I suspect the system is low on hydraulic fluid or someone has disconnected the cylinders from the top frame.

Worst case is the pump is shot. Pumps and cylinders can be rebuilt by any competent hydraulic service shop. Try filling the reservoir and bleeding out the system first; procedure is, as you guessed, in the Fisher body manual. It can be messy.

The top can be operated with the cylinders unhooked, but an original hydraulic top will be clumsy and usually requires two people to lower and especially to RAISE it. Non-power tops had counterbalance springs to help, but I don't think full-size Oldsmobiles had manual tops after about 1950.

A hydraulic top that ain't working is still easier to live with than the 71-76 all-electric scissor top.