OJT exists within your organization. In fact, the majority of employee job knowledge is gained through OJT. Formally or informally, one way or another, experienced employees share "tribal knowledge" with less experienced employees every day. It happens with or without a formal OJT program and with or without managerial/supervisory involvement. 
 

Structured OJT is planned, controlled training. Unstructured OJT is unplanned, uncontrolled training (which, unfortunately, is the most prevalent form of OJT found in organizations). Both forms of OJT have associated costs, but structured OJT is proven to be much more effective in improving performance. If you want efficient, consistent training results (which of course you do), you have to control the WHAT, WHEN, HOW, WHERE, and WHO of training. That's your job, and it goes far beyond merely assigning one employee to train another employee. 

Before we examine the "structure" in structured OJT, remember one thing. You need to analyze job requirements and the performance levels of your employees before you select OJT as your training vehicle. There are many ways to improve performance - OJT is only one option. See the topic - Performance Assessment 

Structured OJT contains several elements:  

1. Job Task Analysis - Determine the job tasks that need to be learned/trained. 
2. Trainer Selection - Select the best person to be the OJT trainer. What does best mean? See the briefing sheet - How do I select an OJT trainer? 
3. OJT Design/Assembly - Design/develop the OJT materials. 
4. OJT Delivery - Conduct structured, planned training with the trainee. 
5. OJT Evaluation - Determine whether the training has been effective. 

How many of these steps does your organization perform? Which steps are done only minimally or not at all? When well-planned and skillfully controlled, OJT is a powerful training method that can yield incredibly big dividends. Like any form of training, it's an investment. Invest wisely.