Sharepoint - It is possible to run a timer job in Office 365 online?

Any type of feature that requires access to Central Administration (timer jobs, etc.) or equivalent server side changes will not be available to the subscriber of Office 365 as the farm may be shared. Please take a look at the links below which may further give you an idea to do your requirement.

Service Descriptions for Office 365 for Enterprises:

http://www.microsoft.com/en-us/download/details.aspx?id=13602

Developing for SPO with Sandbox Solutions:

http://msdn.microsoft.com/en-us/office365trainingcourse_session-2_unit.aspx

http://msdn.microsoft.com/en-us/magazine/ee335711.aspx

Custom workflows with SharePoint Designer 2010:

http://office.microsoft.com/en-us/sharepoint-designer-help/CH010373544.aspx

SharePoint Online for Office 365 Developer Guide:

http://www.microsoft.com/downloads/en/details.aspx?FamilyID=4387e030-73dc-48e7-ac95-abc043b9335a

http://msdn.microsoft.com/en-us/library/hh147180.aspx

Source

It isn't possible to create a timer job on SharePoint online or anything that uses SSOM (Server-side object model).

But you can create a Windows Azure Timer Job and use CSOM (Client-side object model) as an alternative to a full thrust timer job.

SharePoint Timer Jobs running as Windows Azure Web Jobs: http://blogs.msdn.com/b/richard_dizeregas_blog/archive/2014/04/07/sharepoint-timer-jobs-running-as-windows-azure-web-jobs.aspx