If by offertory prayers, you mean the blessing of the gifts (“Blessed are you, Lord God of all creation…”), those are required and cannot be omitted.
Those can, however, be done “secretly” - that is, barely at all voiced. The entire canon can be; it should not be, but can be, save for the people’s public prayers as part of it - the our father, and the I am not worthy…
The following parts may be omitted on weekday masses for non-solemn holy days:
The Gloria #53
The Homily #66
The Creed #68
The universal prayer aka Prayer’s of the Faithful #69*
The individual sign of peace (but not the rite of peace as a whole)**
(If only one person is present) The Ite Missa Est (THe Mass has Ended)*** #272
The following parts may be done “quietly”:
The Gospeler’s prayer before and after the Gospel reading.
The Benedictus, et domine (Blessed are you, O Lord) #142
The priest’s prayer for communion #156
The priest’s prayer at reception: Corpus Christi custodiat me in vitam aeternam (May the Body of Christ keep me safe for eternal life) #158
Certain parts may be substituted for just cause:
Apostle’s Creed for the Credo Romanum
Nicene-Constantinopolitan Creed for the Credo Romanum in languages where the Romanum has issues. (its two words difference, people.)
The Baptismal Promises (When baptisms/confirmations are done at mass)
The use of incense is not required under the current missal, but is always permitted.
- It should not be, but can be, omitted when the mass is public.
** the individual conferences can dictate a sign be given, and how it is done.
*** There are other changes as well in such cases.