Early versions of OpenFlow used a fixed length structure for match statements, and was therefore pretty inflexible. As of OpenFlow 1.2, a TLV structure was adopted, which allows matches to be added in a much more modular way. These are called OpenFlow Extensible Match, or OXMs. In short, these are the fields that the specification allows you to match on. OXM should be thought of as a new format for describing field matches.
Section 7.2.3.7 in the OpenFlow 1.3.2 specification describes all of the OXMs you can match on in this version.
OXMs were initially referred to as NXMs, which stands for Nicira Extensible Match (Nicira obviously now a part of VMware) http://openvswitch.org/slides/NXM-110815.pdf