At what stage one should apply for an OPT job?

One should start looking for OPT job at least 30-60 days before your end of academics.

You can apply for OPT from 90 days before your end date, until 60 days after your end date. USCIS must receipt (not receive) your application no later than 60 days after your I-20 end date.

sooner the better.

When are you I-20 end date?

You should have a job offer first.