Hi,
O_RDONLY means that agent wants read only access to the file. Sharing mode is controlled by third argument, which is set to _SH_DENYNO, meaning that agent allows both read and write shared access to the file. However, the problem may be with the application writing to the file - if application requests exclusive access (i.e. not permitting read access by others), it will not be able to open file for writing. We cannot do anything about it. If you can control the application in question you can change it or request application developers to allow shared read access to the log.
Best regards,
Victor