Can diodes be used as an OR gate when I use RTL for other gates?
Yes, in fact this is exactly what was used in the D17B guidance computer for the original Minuteman I missile:


  "The D-17B contains 1,521 transistors, 6,282 diodes, 1,116 capacitors,
  and 504 resistors.
  
  The design specifications of the D-17B required very high
  reliability. This was achieved by using DRL (diode-resistor) logic
  extensively and only using DTL (diode-transistor) logic where gain or
  inversion was required in this fully solid-state computer. In the
  early 1960s when the D-17B was designed, transistors were not as
  reliable as they are now, thus the designers used transistors only
  when necessary."


DL (Diode Logic) aka DRL (Diode Resistor Logic) can be used to create various logic gates such as AND, OR, OR-AND and AND-OR, but because it is made of of only passive components, the signal degrades fairly rapidly (due to the diodes, you lose 0.7v of signal for each stage).  Also, due to the lack of an active element, it cannot perform inversion (so gates like NAND and NOR are not possible).  Therefore, for any but the most tirivial  circuits, they must be followed by a DTL (Diode Transistor Logic) or other active stage including one or more transistors.