What is the meaning of '脱产培训‘？ Is '脱产培训‘ the opposite of '在岗培训‘ that is to say ’off the job training' compared to 'on the job training'?
Not exactly.

Long answer:

According to 现代汉语规范词典 / The Standard Dictionary of Contemporary Chinese
Copyright © 2010, 2012 Oxford University Press and Foreign Language Teaching and Research Publishing, Co., Ltd. and 牛津英汉汉英词典 / Oxford Chinese Dictionary
Copyright © 2010, 2013 Oxford University Press and Foreign Language Teaching and Research Publishing Co., Ltd.


  脱产 tuōchǎn
  [动]脱离原来从事的直接生产岗位，担任非生产性工作或专门学习；也泛指脱离原来的工作岗位，担任其他工作或专门学习。
  基层干部大都不脱产 | 脱产进修
  
  脱产 tuōchǎn
  [动]
  be released from one’s regular work to take on other duties
  脱产进修
  go on block release
  脱产学习
  be given study leave


In real life, 脱产培训 could mean both quitting your job and take a training or suspending your work duties and take a training.

I assume off the job training means a training you take not as part of your job. And a on job training is a training as part of your job.

But for 脱产培训, if you quit your job and take a training, it certainly is an off-the-job training. But not all off-the-job trainings are 脱产培训 since you can have a job at day and take some personal training at night which should still be counted as off-the-job training even you have a regular work duty.

On the other hand, if your normal duties are suspended and be provided a training opportunity by your company, the training still counts as 脱产培训 but not off-the-job training. Because the training is part of your job, it is on-job-training.

A bit confusing. To sum up, 脱产 concerns more about whether or not your are released from production, i.e your primary duty, not whether or not it is related to your job or organisation.