It depends on what you call "to schedule". OpenERP supports several concepts:

• Calendar of Meetings: each employee has its own calendar, and is free to record any meeting. Leave requests, once validated, appear automatically in the calendar of meetings.
• Leave requests: it's a calendar that only includes "leave requests" -> that is when they plan to not come and is not related to what they do.

Every application (like projects, CRM) may have other calendars. As an example, in the project application, you can have the calendar of tasks that shows who should do what.

In all these calendars, it is possible to create entries for others: you can create a meeting for a colleague, you can create a task and assign it to a colleague,...