555 as a Digital Coupler
Probably you don't know/understand what isolation means when dealing with optocouplers/optoisolators.

It doesn't mean that there is some sort of circuit buffering separating the two circuits. It really means electrical isolation, i.e. there is no current flowing from one part to the other (an optocoupler does that).

An optocoupler is essentially an LED and a photodiode (or a phototransistor) optically coupled in the same package. The LED and the photodiode don't share any electrical connection. They only "communicate" through the light generated by the LED. Therefore the "input section" (the LED) can be driven by a circuit which is completely isolated from the circuit connected to the "output section" (the photodiode/phototransistor). 



This allows two circuits that must remain electrically isolated to communicate.

So the answer is no, an NE555 cannot substitute a circuit which performs electrical isolation.

If you're desperate or don't need specific performance/reliability you could try building a poor man's optocoupler. And here is another attempt.

Important Note: if your circuit needs an optocoupler for stringent safety reasons, using one of those home made one could pose some serious safety risks, so be warned!

That is, I wouldn't trust a mains-powered heart-beat detector using a home-made optocoupler to be used on my chest!!!