Well, actually you don't, but it really depends on how your system is configured. Or it might even be more accurate to say, you need it, but a good IT implementation will hide that detail from you. (BTW: If you need an OID for the NQF Measure authoring tool, read through to the end)
What is an OID? It's an IT thingy that allows you to make sure that the identifiers you use in your organization are distinct from those used by another organization.
Why do you need one for Meaningful Use? I'm assuming that somebody has told you that you need one. In that case, what you most likely need is an organizational OID assigning authority root from which your application will then create additional OIDs for patient, visit and other identifiers that are created by that application. If you want to know more about the technical details, you can read the OID Implementation Guide (if you are an HL7 Member).
To get an OID free from HL7, traverse this link (or have your experienced IT person do it). Click on the link in the upper right hand corner. Fill out the first page with their name and e-mail address. On the next page, fill out the contact information with someone who you expect to be around for a while, and who gets technology, and can deal with technical questions about IT.
When you get to the question about Internal/External, what you want is an Internal OID (because if you already have an OID, you know more than enough about OIDs than anyone who needs to read this far). Accept the default settings. You want a "Type 3" OID (Root to be a Registration Authority), which you can then "partition" into new OIDs according to the OID Implementation Guide linked to above.
This takes all of two minutes to do. Once you press submit, you will be given your OID.
When you are done, you will be taken to a page that gives you your OID, and gives you a long key which you can use to edit the details.
A smart IT implementer will take care of these details for you, and just ask you for the name of the person who should be the contact person for your OID, and will configure your system to deal with that new OID. And if they are really smart, they will simply ask whether you have an OID you want to use, or whether they should take care of that detail for you.
OK, now for the additional instructions for those needing an OID for the Measure Authoring Tool. Follow the same instructions above. Pick a number from 20 to infinity. Add a dot and that number to the end of your OID. That is your Value Set OID Root. Now pick another number, and add a dot and that number to the end of the OID you just created a moment ago. That's your Value set OID. Or read this.