Oracle Upgrade path as Per Metalink Note 2189854.1  ...

Database Server Upgrade/Downgrade Compatibility Matrix (Doc ID 551141.1) Database Server Upgrade/Downgrade Compatibility Matrix (Doc ID 551141.1)

 OS certification  for database 

https://support.oracle.com/epmos/faces/CertifyHome

Direct Upgrade to 12.2:

Source Database	Target Database
11.2.0.3 and higher	12.2.0.x
12.1.0.x (12.1.0.1 - 12.1.0.2)	12.2.0.x

Indirect Upgrade to 12.2:

Source Database	Upgrade Path	Target Database
7.3.3 (lower)	7.3.4 --> 9.2.0.8 --> 11.2.0.3 or higher	12.2.0.x
8.0.5 ( or lower )	8.0.6 --> 9.2.0.8 --> 11.2.0.3 or higher	12.2.0.x
8.1.7 ( or lower )	8.1.7 --> 9.2.0.8 --> 11.2.0.3 or higher	12.2.0.x
9.0.1.3 ( or lower )	9.0.1.3 --> 9.2.0.8 --> 11.2.0.3 or higher	12.2.0.x
9.2.0.7 ( or lower )	9.2.0.7 --> 11.2.0.3 or higher	12.2.0.x
10.2.0.4 ( or lower )	10.2.0.4 --> 11.2.0.3 or higher	12.2.0.x
11.1.0.6	11.1.0.6 --> 11.2.0.3 or higher	12.2.0.x
11.2.0.1	11.2.0.1 --> 11.2.0.3 or higher	12.2.0.x

       Minimum Version of the database that can be directly upgraded to Oracle Database 19c

Source Database	Destination Database
18 (all releases)	19c
12.2.0.1	19c
12.1.0.2	19c
11.2.0.4	19c

Direct upgrade to 19c is not supported to rest of the database releases/versions which are not mentioned above.
So first Upgrade to an intermediate Oracle Database release which  can be then directly be upgraded to 19c.

                                                                                       Minimum Version of the database that can be directly upgraded to Oracle Database 21c
                        Source Database
         Destination Database
                                       19c
                                  21c
                                       18c
                                  21c
                                       12c Release 2 (12.2)
                                  21c
Direct upgrade to 21c is not supported to rest of the database releases/versions which are not mentioned above.
So first Upgrade to an intermediate Oracle Database release which can be then directly be upgraded to 21c.

Note: 23ai released only for Oracle Database Cloud services & Exadata
23ai is still not released for On-Premises Server.Once it's released the information will be updated.

                                                                                           Minimum Version of the database that can be directly upgraded to Oracle Database 23ai

Source Database	Destination Database
21c	23ai
19c	23ai

Direct upgrade to 23ai is not supported to rest of the database releases/versions which are not mentioned above.
So first Upgrade to an intermediate Oracle Database release which can be then directly be upgraded to 23ai.

Helpful metalink note ..

Note: 2173141.1 Complete Checklist for Manual Upgrades to non-CDB Oracle Database 12.2

Note: 2173144.1 Complete Checklist for Manual Upgrade for Multitenant Architecture Oracle Databases from 12.1. to 12.2.

Note: 2189854.1 Complete Checklist for Upgrading to Oracle Database 12.2 using DBUA for Known Issues and Alerts

Note: 2239820.1 12.2.0.1 Base Release – Availability and Known Issues