I am able to get all objects which are related in opportunity object by looking into opp-->Fields.I can see related object lookup(Object name).but how can i know other objects which are using opportunity lookup/master-details relationship. Please suggest
For this you have to understand the SFDC Object Relationship structure.
When you create a master-detail or a lookup relationship, you create it on the child object and relate it to the master object. So the details of the relation is listed under the child object's field list.
So any relation in which Opportunity is the child, you will find that in the opportunity's field list (eg. Opportunity to Account). So if you want to see the relations in which Opportunity is the parent, you should look into the child object's field list.
EDIT: In case you are not sure which all child object opportunity have, you can go to Setup->Schema Builder and Add opportunity and then add all other custom objects you want to check with, if there is a relationship, it will show it using a connection.
Mohit's answer on this question is also a very good way using workbench.