Few things:

In order to master Purchasing, you should also understand the functional of Payables. Otherwise you would not have a complete picture of what will eventually happen to the POs.

In order to master Order Management, you should also understand the functional of Receivables. Otherwise you would not have a complete picture of what will eventually happen to the orders.

In order to understand Payables and Receivables (also Purchasing and Order Management) you must have a strong foundation of:
1- "organization structures" in Oracle including Business Groups, Ledgers, Legal Entities, and Operating Units.
2- Chart of Account Structure, Balancing Segment Values, Ledger Options etc.
3- Basis accounting concepts including debit credits, journals, postings etc.

So the road map would be:

1- Foundation videos under GL section that addresses "Organization Structures" listed above.

2- Videos related to accounting, journals, and posting.

* You can skip GL videos talking about rest of the GL e.g. Mass Allocations, Budgets, Recurring Journals etc.

3- Study Payables

4- Study Purchasing

5- Study Receivables

6- Study Order Management

If you wish to speciliaze in Purchasing and Order Management by skipping Payables and Receivables, you can do that. But I will not be comfortable myself following this approach.

Usually following modules go together in terms of specializations:

1- GL - AP - PO - (CE)
2- GP - AR - OM - (CE)

If you wish to discuss this further please open a new post as this post is related to some other question by other members.