[scroll down for a description of the repository content] **Abstract** Background: Despite evidence of selective outcome reporting across multiple disciplines, this has not yet been assessed in trials studying the effects of exercise in people with cancer. Therefore, the purpose of our study was to explore prospectively registered randomised controlled trials (RCTs) in exercise oncology for evidence of selective outcome reporting. Methods: Eligible trials were RCTs that 1) investigated the effects of at least partially supervised exercise interventions in people with cancer; 2) were preregistered (i.e. registered before the first patient was recruited) on a clinical trials registry; and 3) reported results in a peer-reviewed published manuscript. We searched the PubMed database from the year of inception to September 2020 to identify eligible exercise oncology RCTs clinical trial registries. Eligible trial registrations and linked published manuscripts were compared to identify the proportion of sufficiently preregistered outcomes reported correctly in the manuscripts, and cases of outcome omission, switching, and silently introduction of non- novel outcomes. Results: We identified 31 eligible RCTs and 46 that were ineligible due to retrospective registration. Of the 405 total prespecified outcomes across the 31 eligible trials, only 6.2% were preregistered complete methodological detail. Only 16% (n=148/929) of outcomes reported in published results manuscripts were linked with sufficiently preregistered outcomes without outcome switching. We found 85 total cases of outcome switching. A high proportion (41%) of preregistered outcomes were omitted from the published results manuscripts, and many published outcomes (n=394; 42.4%) were novel outcomes that had been silently introduced (median, min-max=10, 0-50 per trial). We found no examples of preregistered efficacy outcomes that were measured, assessed, and analysed as planned. Conclusions: We found evidence suggestive of widespread selective outcome reporting and non-reporting bias (outcome switching, omitted preregistered outcomes, and silently introduced novel outcomes). The existence of such reporting discrepancies has implications for the integrity and credibility of RCTs in exercise oncology. **Content of the repository** This repository contains all data, code, and materials to reproduce our analyses and, we hope, to replicate our method. **Preregistration:** The folder Preregistration contains - The OREO [preregistration][1] document which was preregistered on November 19, 2021. **Original data:** The folder original data contains - A [meta-data][2] folder that contains a file explaining the data we have provided. - The full dataset, as three Excel files: 1) [OREO retrospective trials data][3]; 2) [OREO Trials without results publications data][4]; 3) [OREO main analysis data][5] for all eligible trials. - A [document][6] with a link to a [Google Doc webpage][7] that allows readers to access the data we extracted from each of the 31 included studies in OREO. **R Markdown file:** The folder [R Markdown file][8] contains - The [R Markdown file][9] used to produce the analysis including all data in tables and figures (both in the main manuscript and supplementary materials). **Manuscript:** The folder Manuscript contains - A Manuscript folder containing a document with a link to the pre-printed manuscript on MedRxiv. - A Study figures folder containing the jpeg files for the six figures included in the main manuscript and a [PowerPoint][10] file containing all six figures. **Supplementary material:** The folder Supplementary material contains - A [Supplementary material][11] document that which contains all supplementary tables (1-10) and figures (1-7). [1]: http://osf.io/dtkar/ [2]: http://osf.io/8rd6s/ [3]: https://osf.io/qrwjy/ [4]: https://osf.io/qpeus/ [5]: https://osf.io/exyms/ [6]: https://osf.io/sebk3/ [7]: https://bit.ly/3qFkcro [8]: https://osf.io/63bjw/ [9]: https://osf.io/63bjw/ [10]: https://osf.io/hkze5/ [11]: https://osf.io/ga4sc/