standard is winch
on the mast
which really means it is a two person job to raise the main. You can bring halyard
back to electric winch
assuming a fixed mast
so it can be operated by one person in theory.
Theory and practice however, can deviate. In flat seas I would have no problem bringing the boat into the wind
and raising the main. In fact I do it regularly. In lumpy seas it becomes problematic as you need to keep an eye on steering
the boat, raising the main and watching it does not get caught in the lazyjacks as it jumps around. When inevitably it gets caught it is difficult to lower and control at the boat at the same time.
It stands to reason that the smaller the main the easier it is to sail single handed.
I have never sailed an O51 but I have sailed boats of a similar size and I would not relish sailing it single handed, but I suppose everything is possible.