The simple answer to your question is this: you should always make candidates follow your standard process for expressing interest. If you generally require that candidates complete an online application, then you should ask candidates who are at your hiring event to complete the online application. It's not clear from your post above why you are doing "an in person hiring event," and why this would necessitate making immediate offers to candidates. There certainly are circumstances in which companies want and need to make immediate hires. For example, if you have a huge new order for products or services that requires you to immediately add employees, an in person hiring event may make sense. However, if your company's standard hiring process is to require candidates to apply online after posting an opening, then there should be some well defined reason for deviating from this process. One way to think about this situation is to consider it from the perspective of someone from a governmental agency who may be asked to determine whether your hiring process discriminated against some protected class. The representative of the government agency is going to want to compare the credentials of candidates who expressed interest against the credentials of candidates who were hired, and is going to want to understand how your selection process worked and who was involved in the process. If hiring managers are allowed to deviate from your standard selection processes and make offers to people on the spot at your in person hiring event, they will need to be able to clearly state why the persons selected are more qualified than others. A governmental representative will not accept the idea that the pressing need for people allowed you to hire demonstrably less-qualified candidates. If you still want to do an in person event and you don't want to make people go home and apply online, then you may want to consider having kiosks available at the event so that people can apply while they are there. That way, at least you'll have collected all the information you need to determine who the best qualified candidates are. Then, if you want to tell some candidates that you are seriously considering them for hire, you'll be able to quickly look at the qualifications of other candidates to ensure you are hiring the best qualified people. I understand the need to quickly hire people when you have pressing business. From a compliance perspective, though, it's important to recognize that the government will always expect that you can prove you hired the best qualified person. Making offers on the spot has the potential to leave you vulnerable if you can't provide this proof.
You can use this OFCCP audit checklist to ensure you're doing what is required to maintain OFCCP's regulations including VEVRAA, Section 503, and EO 11246. Or request a demo to streamline your compliance and recruiting efforts.