The usual reason for `oerrs' is late collisions or loss of carrier
sense.  Ethernet devices do not detect any other errors during
transmission.  If you enable full-duplex mode, they can't even do this
much (because the receive line is completely independent), so `oerrs'
will never increase.

What this probably means is that some device transmitting to your
machine is in full-duplex mode, and thus your card is detecting the
transmissions not as receivable packets but as collisions, because
it's in half-duplex mode.