There are several steps that should be taken when troubleshooting connection issues to Oracle databases. Firstly you should follow the guide on installing Oracle in a distributed environment:

Ensure that you have installed the 32bit version of the Oracle client tools on the CRM server and that the Oracle listener is active. You must also configure the Net Configuration Assistant on the CRM server to establish the correct connection with the listener. If you are experiencing issue connecting to the database after this you should try the following:

Open command prompt as administrator and try the following commands:

tnsping <hostname>
ping <hostname/ip>
slplus <username>/<password>@<service-name>

Issues in any of the above commands could indicate potential network problems between the CRM and Oracle servers. You need to ensure that the CRM server and Oracle server can communicate at the network level and that the Oracle listener is active.

In some instances there can still be errors even if SQLplus, tnsping and ping are all working correctly. The next step to check is that the OraOLE11.dll file is registered. The OLEDB client is responsible for the COM communication which is a vital part of Sage CRM. To do so open command prompt as administrator-> cd to <ORACLE-HOME>\bin
Check your version of the OraOLEDB dll file. For Oracle 11 it should be OraOLEDB11.dll so run:
regsvr32 OraOLEDB11.all

If you get an issue while registering it, it may be due to not opening the command prompt as an administrator.

More information:
These issues may occur either when first installing/upgrading Sage CRM or due to some network changes. When running the installation you may see the following error: "Provider cannot be found, it may not be properly installed" which will also appear in the logs. Reviewing the eware and sql logs are a vital step of the analysis, you must check what CRM has registered as part of the problem as this will enable you to further research what is causing your connection issue.