With SAPUI5 and OpenUI5 we provide two deliveries of our UI development toolkit. Both are very closely related, but have their differences.
The main difference is the license.
OpenUI5 is Open Source, free to use, released under the Apache 2.0 license. Since we also use many Open Source libraries, we try to return the favor and also benefit from the experience and knowledge of developers all over the world.
SAP HANA Cloud Platform
SAP NetWeaver 7.4 or higher (included in the UI technologies (SAP_UI) component)
User interface add-on for SAP NetWeaver for SAP NetWeaver Application Server 7.3x
The easiest way to get an overview of which libraries are delivered is to have a look at the API Reference of the each Demo Kit. You'll see that the list of libraries in SAPUI5 is much longer... which in no way means that OpenUI5 provides just a very limited scope!
Most importantly, the core containing all central functionality and the most commonly used control libraries is identical in both deliveries. (For example, sap.m, sap.ui.layout, sap.ui.unified, and the more desktop-focused libraries sap.ui.commons, sap.ui.ux3.)
So OpenUI5 also gives you all the important features needed to build feature-rich Web applications.
The additional libraries in SAPUI5 include more controls on top, like charts, and SAPUI5 also lets you use 'smart controls', for example, which are controls that are automatically configured by OData annotations from the back end. The exact feature range of SAPUI5 also depends on the platform you're using. For example, you can only use the ABAP repository with SAP NetWeaver and not on SAP HANA Cloud Platform.
For the SAPUI5 resources, check your platform installation.
Technically, you can switch between OpenUI5 and SAPUI5 (providing you have the respective license), e.g. if you want to use the SAPUI5-specific features.
Just check first which SAPUI5 version you need, because the version numbering is not 100% the same. The OpenUI5 version number must match the Core Version number of the SAPUI5 version. You can find this information in the technical info dialog (Ctrl + Alt + Shift + P).
If you're using the content delivery network, you can simply replace the bootstrapping reference to https://openui5.hana.ondemand.com/<1.xx.yy>/ with a reference to https://sapui5.hana.ondemand.com/<1.xx.zz>/. For more information, see Variant for Bootstrapping from Content Delivery Network.
For all other cases, replace the runtime. Since the technical names (of controls, libraries, etc.) and APIs are the same in both OpenUI5 and SAPUI5, the code will still work and you can start enhancing it directly.