Ben Oss’s other ridge goes south west to a col at the head of Coire Laoigh with a height of 687 metres which links to Ben Lui. Ben Oss’ southern slopes drop less steeply to Glen Falloch and the land at the head of Loch Lomond. Ben Oss lies on the drainage divide of Scotland, all rainfall that falls on the northern slopes drains to the Cononish glen which forms the headwaters of the River Tay and eventually goes to the east coast. Drainage from the southern slopes goes via Glen Falloch and Loch Lomond to reach the sea at the west coast at the Firth of Clyde.