I'll be having an early morning flight (6am) from Porto airport (OPO) with Lufthansa.
Will I be able to drop my luggage (assume I checked in online) the night before?
"We are not offering a late night check in in Porto, please bring your luggage in the morning. /Nina"
So no, you need to get rid of your luggage on the morning of your flight.
For Lufthansa in general, the website http://www.lufthansa.com/de/en/Check-in-Information allows you to select the airport you are flying from to get information about check-in possibilities. If late night check-in is possible, it is mentioned under "On-airport services". For Porto, it is not available (I double checked by selecting Frankfurt, where I know it is available, and it showed up)