For a full walkthrough, please see this overview of OCM.
A campaign can be thought of as a housing for multiple flights from the same advertiser.
To create a campaign, you can follow these steps:
1) Head to the campaign menu and click "New campaign":
2) Give your campaign a name and a start and end date.
3a) If this is a campaign for an advertiser that has already run, then you can simply choose the advertiser from the dropdown menu. Otherwise, click "Create new advertiser" (circled in red below)
3b) Name your new advertiser, select a currency that they will be paying in and then select an IAB category for category separation. You can read more abou category separation logic here.
3c) Once you're happy, click "Create advertiser".
4) Review your campaign details and if it all looks OK - click "Create campaign".