When I link an opportunity to a parent opportunity, how do I see it (and others) within the parent opportunity?

Opportunities do not have a native relationship back to themselves - so I'm guessing you're looking at a custom relationship/relate field that has been added to your system.

If it is a custom many-to-one relationship, you're selecting the parent opportunity on the record view, and then if you click to the parent and scroll down to the list of subpanels, there should be a subpanel called "Opportunities" - showing you each of the child opportunities.

If you do not see that subpanel, it could mean:

1. the subpanel is hidden  - which is easy to check by seeing if you can see this subpanel on the Accounts page - if it doesn't show there, you can show them again using the instructions here: https://support.sugarcrm.com/Documentation/Sugar_Versions/10.1/Sell/Administration_Guide/Developer_Tools/#Display_Modules_and_Subpanels
2. either a relate field or a 1:1 relationship was used instead of a many-to-one relationship. You cannot change this - you may as well either delete the field/relationship, or just hide it. Worth reading this to understand how they work:https://support.sugarcrm.com/Knowledge_Base/Studio_and_Module_Builder/Introduction_to_Relationships_and_Relate_Fields/#Relate_Fields 

Hopefully that addresses your question!

Obviously, I don't know anything about your business/context, but something to be mindful of: linking opportunities to a parent opp can have some interesting impacts upon your pipeline - you need to be careful you're not double counting revenue. Although there are plenty of good reasons to link opportunities together (e.g. they all relate to the same project), I would generally not use a parent opportunity to do so, as the risk of reporting potential revenue incorrectly is too high. I'd group it by some other record instead, based upon the given business need.

Opportunities do not have a native relationship back to themselves - so I'm guessing you're looking at a custom relationship/relate field that has been added to your system.

If it is a custom many-to-one relationship, you're selecting the parent opportunity on the record view, and then if you click to the parent and scroll down to the list of subpanels, there should be a subpanel called "Opportunities" - showing you each of the child opportunities.

If you do not see that subpanel, it could mean:

1. the subpanel is hidden  - which is easy to check by seeing if you can see this subpanel on the Accounts page - if it doesn't show there, you can show them again using the instructions here: https://support.sugarcrm.com/Documentation/Sugar_Versions/10.1/Sell/Administration_Guide/Developer_Tools/#Display_Modules_and_Subpanels
2. either a relate field or a 1:1 relationship was used instead of a many-to-one relationship. You cannot change this - you may as well either delete the field/relationship, or just hide it. Worth reading this to understand how they work:https://support.sugarcrm.com/Knowledge_Base/Studio_and_Module_Builder/Introduction_to_Relationships_and_Relate_Fields/#Relate_Fields 

Hopefully that addresses your question!

Obviously, I don't know anything about your business/context, but something to be mindful of: linking opportunities to a parent opp can have some interesting impacts upon your pipeline - you need to be careful you're not double counting revenue. Although there are plenty of good reasons to link opportunities together (e.g. they all relate to the same project), I would generally not use a parent opportunity to do so, as the risk of reporting potential revenue incorrectly is too high. I'd group it by some other record instead, based upon the given business need.