You can use the Project Formats inspector to define how your project displays units of time and currency, accessible via the Inspectors menu.

Use the Dates section to choose whether to include seconds, the time of day, or both in your displayed dates.

Choose a standard Currency format from the list, or just type the number 1234.56 in the format of your choice.

Time Units selected here are used to display durations and effort amounts. You can plan in hours, days, or whatever works best for you. Note that, in duration fields, if you enter a number without any units, OmniPlan assumes that you mean the smallest unit that is enabled here. Units of time (Duration and effort) are designated throughout OmniPlan by the abbreviations below:

If you want to go further than just customizing the display of durations, you can force OmniPlan to schedule in whole hours or whole days with the scheduling granularity setting in the Project Information inspector. Note that the date headers in the Gantt chart and resource timelines can also be customized with the Display preferences.