The core game of OpenTTD is released under the GNU General Public License (GPL) version 2.
The OpenGFX baseset graphics pack is released under the GNU General Public License version 2.
The OpenSFX baseset sound effects pack is released under the Creative Commons Attribution-ShareAlike 3.0 Unported license.
The OpenMSX baseset music pack is released under the GNU General Public License version 2.
All the above licenses permit use of the software for any purpose, including commercial purposes.
Following is a statement of intention from the developers. This is not a legal statement. If any questions arise from this statement, please consult the license text of the involved software and game data.
General permission for the unmodified game
It is the intention of the development teams behind OpenTTD and the open baseset game data packs that any Let's Play, review, and other similar content, is permitted with no limitations. Monetization and commercial use of the game and game data is permitted without limitations.
Note that regardless of this statement, any distribution of the game and game data files still falls under the restrictions of the respective licenses.
Note that this statement does not imply any endorsement from any of the developers of OpenTTD or the baseset game data towards any content creators. Any views expressed by content creators in content using the OpenTTD game and baseset game data packs are purely those of the content creator.
Non-coverage of use with other baseset data
Note that the above statement of general permission only applies to using the OpenTTD core game with the OpenGFX graphics baseset, and optionally the OpenSFX sound effects and OpenMSX music basesets.
Use of any other baseset data, including but not limited to the data files from Transport Tycoon (Deluxe), is not covered by the above statement of general permission, and you will need to verify with the copyright holder for the baseset data before using it in content creation.
Non-coverage of use with add-on game data
Note that the above statement of general permission only applies to using OpenTTD without any add-on game data. Add-on game data can include NewGRF modifications to graphics and game rules, scenario and heightmap files that offer a designed world to play in, AI scripts that offer computer-controlled opponents, Game Scripts that offer altered game rules, as common examples.
Add-on game data can be offered under a variety of different licenses, and some may not allow all uses. Please verify with the copyright holder of add-on game data before using it in content creation.