Pessimistic locking is not an option with Odoo, being quite impractical in a stateless web-based environment.

Odoo implements Optimistic Concurrency Control:

For every object, there is a virtual __last_update field that clients can requests explicitly as part of regular read() calls, on any model. The value is automatically inferred from the write_date and create_date fields that the framework maintains on any record by default. When the client later makes a write() call on the same record, it can pass an extra context structure containing the original __last_update value, and the system will automatically raise an error if the record has been updated in the meantime. See the _check_concurrency() method for more details.

See http://stackoverflow.com/questions/18960668/no-concurrency-check-when-updating-records