My collaborator uses a Mac and cannot read my odt text files.

Is there anything i can do to dmodify the odt text file before I sent it OR is there an app he can install on his Mac to read my odt file?

Does you collaborator need to only read your file, or does he need to modify it and send it back to you?

If he only needs to read it, you can easily export your document as a portable document format (PDF) file and send that to him. If he needs to modify and work with the file, he can download and install LibreOffice for Mac OSX.

There is LibreOffice on mac OS. Your collaborator can download it form Donate to LibreOffice | LibreOffice - Free Office Suite - Based on OpenOffice - Compatible with Microsoft and then install it. Then he will can read your ODT files