You will be sent the dress code with your tickets. Normally there are two "formal" nights each week when men wear dinner jacket or a dark suit. The rest are a mixture of informal (shirt/ tie or jacket - no shorts) and casual (when in port and on first and last night of the cruise) casual (no tie needed but no shorts. In addition there will be theme nights eg tropical, red/white/blue, Western.
Most of this info is in the Fred. Olsen brochure and in the website (fredolsencruises.com.uk).
Hope this helps.