Excerpts from the
Oracle 18c XE Licensing information:
OTN Developer License for XE (Windows)
"We grant you a nonexclusive, nontransferable limited license to use the programs for: (a) purposes of developing, prototyping and running your applications for your own internal data processing operations; (b) you may also distribute the programs with your applications; (c) you may use the programs to provide third party demonstrations and training; and d) you may copy and distribute the programs to your licensees provided that each such licensee agrees to the terms of this Agreement. You are not permitted to use the programs for any purpose other than as permitted under this Agreement. Program documentation is either shipped with the programs, or documentation may accessed online at"
Free to Use License (Linux)
"Oracle grants to You, as a recipient of this Program, a nonexclusive, nontransferable, limited license to, subject to the conditions stated herein, (a) internally use the unmodified Programs for the purposes of developing, testing, prototyping and demonstrating your applications, and running the Programs for your own internal business operations; and (b) redistribute unmodified Programs and Programs Documentation, under the terms of this License, provided that You do not charge Your end users any additional fees for the use of the Programs."
"Oracle grants You a nonexclusive, nontransferable, limited license to internally use the Programs, subject to the restrictions stated in this Agreement, solely for your business operations and any third party training as part of such business operations. You may allow Your Contractor(s) to use the Programs, provided they are acting on Your behalf to exercise license rights granted in this Agreement and further provided that You are responsible for their compliance with this Agreement in such use. You will have a written agreement with Your Contractor(s) that strictly limits their right to use the Programs and that otherwise protects Oracle’s intellectual property rights to the same extent as this Agreement. You may make copies of the Programs to the extent reasonably necessary to exercise the license rights granted in this Agreement. You may make one copy of the Programs for backup purposes."
"We grant you a nonexclusive, nontransferable limited license to use the programs solely for your internal business operations subject to the terms of this agreement and the license grants and restrictions set forth with your licensing of such Oracle database programs. We also grant you a nonexclusive, nontransferable limited license to use the programs for purposes of developing your applications."
For more information, please see the full documents (links provided) and ask your lawyer or Oracle for an official statement from Oracle.