Question: I need to understand the fine details, possibilities and pricing of Salesforce OEM model. I can't find an official link about this on salesforce and google, only link available is this 2008 pdf : <http://www.developerforce.com/tdf/2008/april/Force_ISV_Partner.pdf>

Please share any good post/article on this.

Answer: 
We did something similar, however we created a specific boolean field for this purpose: "locked\_\_c".

The benefit of using this field instead of a 'first closed date', is it can be used in multiple ways and remains true to its meaning. 
For example, you may want to lock an Opportunity for a reason other than it was closed 90 days ago, such as based on the Amount value or the Client. By having a generic 'locked' field, the same field is used and it's easy to report on, otherwise you'd have to create multiple fields to achieve the same result.

Our use was slightly different. We wanted certain profiles to have the ability to edit a record in a very small number of situations. We made it possible for those profiles to untick the 'locked' field, then edit it, and workflow would re-tick the field.  

We wanted these users to be able to help themselves, but we also wanted to monitor it, so we had emails sent to the sys admins whenever the 'locked' field was unticked for auditing purposes. This approach worked out quite well as the users could perform their tasks, but we could also control it.
The best non-record type solution I can think of is to have a time-based workflow change the owner. You could then use sharing rules to give other users read-only access to the records.

In addition, while extra record types might be cumbersome to maintain they still might prove to be the simplest and cleanest solution in the end.