I agree with Icebear.  The last time I took a cab from Frankfurt to Oase it cost about E50.  I don't know if it would be more or less from the airport.  German taxis charge by time as well as distance so exact amount depends on how fast traffic moves.  The last time I took the train it was a little under E4.00 from Frankfurt to Friedrichsdorf.  I always found taxis waiting at the train station there.  Cab fare to Oase was right around E10.  Trains are easy to take with computerized ticket machines in the station with instructions in numerous languages including English.  I would only caution you to hang on to your ticket.  You do not need it to board, but they do do spot checks.  The people checking tickets are usually dressed like street thugs.  
    The cabs at the Friedrichsdorf train station should all know how to get to Oase.  One driver told me he averages three trips a day there.  If you must take a cab from the airport you should also have the address for the driver.  You can find that on the Oase web site.  Get their phone number as well, just in case.  Cabs there all seem to have GPS, so the address should be enough.  Returning, you will need a cab from Oase in any event, either to the Friedrichsdorf train station or the airport.  Just ask the nice lady at the front desk.  She will call one that will pick you up right at the door.
    Have fun, and don't do anything I wouldn't do.