often

ofen

We have mastered the spelling of this word so well, its spelling influences the pronunciation: DON'T pronounce the [t]! This is an exception to the rule that spelling helps pronunciation.

ordinance

ordnance

You may have to use ordnance to enforce an ordinance but you should not pronounce the words the same.

orientate

orient

Another pointless back-formation. We don't need this mispronunciation from "orientation" when we already have "orient." (See also "interpretate")

ostensively

ostensibly

Be sure to keep your suffixes straight on this one.

Ostraya

Australia

This pronunciation particularly bothers Australians themselves, most of whom can manage the [l] quite easily, thank you.