From what place will you arrive to Japan?
Have you confirmed that you cannot choose Kansai Airport [KIX]?
(It is less than 1 hour to get from KIX to Osaka City by railway.)
To travel from Narita Airport [NRT] for Osaka City all by bus,
usually you change buses somewhere in Tokyo (#).
(As is mentioned by frog1954,
there is a bus departing from Narita Airport,
on the line which is operated jointly by Chiba Kotsu and Nankai Bus;
however it seems you cannot book there a bus ride through a website in English.)
Personally I cannot recommend a night bus ride just after a long flight to Japan,
especially not for first-time Japan travelers.
The level of quality of expressway bus services
depends on routes, operator companies and price ranges.
(Jumping at a low price may result in putting up with some inconveniences.)
The overnight bus route connecting Tokyo and Osaka City is highly competitive.
(1) Bus ride from Narita Airport (Terminal 1 , 2 or 3)
to Tokyo Station (Yaesu South Entrance) http://accessnarita.jp/en/home/#timetable-title-3386
(2) Walking for about 160 meters between the bus stops https://www.google.co.jp/maps/dir/35.6799036,139.7697009/35.6799073,13...
(3) Bus ride from Tokyo Station (Yaesu Entrance) https://japanbusonline.com/en/CourseSearch/11700010073