Otepää is about 45 km south of Tartu and 250 km from Tallinn. There is no railway. Buses run several times a day from Tartu (about 1 hour) and a couple of times a day from Tallinn (about 4 hours); check Estonian operator GoBus or Lux Express for current schedules.
By car the drive from Tallinn is about 3 hours on the Via Baltica then the Tartu road; from Tartu about 50 minutes through gently rolling farmland. In winter the road can be slow and snow tyres are required by law.