ob-

from the GNU version of the Collaborative International Dictionary of English.

• A prefix signifying to, toward, before, against, reversely, etc.; also, as a simple intensive; as in oblige, to bind to; obstacle, something standing before; object, lit., to throw against; obovate, reversely, or oppositely, ovate. Ob- is commonly assimilated before c, f, g, and p, to oc-, of-, og-, and op-.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• prefix non-productive Against; facing; a combining prefix found in verbs of Latin origin.
• prefix botany Of a reversed shape.
• prefix Internet, informal Obligatory; prepended to the name of a topic being mentioned to avoid accusations of being off-topic.