First round offers expire may 31st, which means that if you don't accept McMaster OT, your completely relying on Western to go through its wait list (Which is what im doing since i didn't get offers anywhere else).
I've called western multiple times to ask if they can give me any info on its wait-list trends and etc. but they wont disclose anything. All I know is that they over accept in anticipation of people not accepting their offers and since they removed English as their prerequisite they have more applicants now and are giving out more offers this year then previous years.
So unless your willing to risk a year off accepting OT would be your best option. Best of luck on whatever you decide though.
BTW im also in the upper third with a sub-GPA of 3.89 so im hoping that Im closer to the top of the wait list