Oce OP33 Reference Manual page 266

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7-20 Envelope Feeder Attachment-B1
Grip the handle and pull out the paper drawer until it stops.
Squeeze the lever on the left guide, as shown below, and pull out the
left guide as far as possible.
Load the envelopes with the printing surface face down, as shown
below. While doing so, keep the receptacle pressed down by
pressing down on the envelopes as you load them.