According to Osho, his 'strange' English arises because he claims not to know any language and speaks spontaneously, unconcerned with grammar or national styles. He talks from the moment, not as an orator, so slips and mixes occur. He calls it 'Rajneesh English' - language used as a vehicle for presence, humor, and truth rather than correctness.
He doesn’t try to speak perfect English; he just talks from his heart in the moment, so it sounds unusual.