The longest words in Oxford Dictionaries are:

• antidisestablishmentarianism – opposition to the disestablishment of the Church of England – 28 letters
• floccinaucinihilipilification – the estimation of something as worthless – 29 letters
• pneumonoultramicroscopicsilicovolcanoconiosis – a supposed lung disease – 45 letters

1. otorhinolaryngological – 22 letters
2. immunoelectrophoretically – 25 letters
3. psychophysicotherapeutics – 25 letters
4. thyroparathyroidectomized – 25 letters
5. pneumoencephalographically – 26 letters
6. radioimmunoelectrophoresis – 26 letters
7. psychoneuroendocrinological – 27 letters
8. hepaticocholangiogastrostomy – 28 letters
9. spectrophotofluorometrically – 28 letters
10. pseudopseudohypoparathyroidism – 30 letters

Credit Source:https://en.oxforddictionaries.com/explore/what-is-the-longest-english-word