I am not sure what has happened but everything was working fine, I then deleted 4 old posts on the site and got an error.

I closed the site and tried to log back in but everytime I try to log in I get the following error message:

Warning: Cannot modify header information - headers already sent by (output started at /home/hospijjj/public_html/jobs/oc-includes/osclass/utils.php:2295) in /home/hospijjj/public_html/jobs/oc-includes/osclass/utils.php on line 2301

Any ideas what this could be about?

it's like you are trying to set headers twice, but it can mean milion of reason, also empty string somewhere outside PHP tag (if I am not wrong).
I would recommend to try disable plugins, maybe it's plugin issue.