I found out the SRX wasn't pushing a default route to the Mikrotik, so I added:
set policy-options policy-statement ospf-default from protocol static
set policy-options policy-statement ospf-default from route-filter 0.0.0.0/0 exact
set policy-options policy-statement ospf-default then accept
set protocols ospf export ospf-default
Now it works, the routing table on my Mikrotik looks like:
[admin@MikroTik] > /ip route print
Flags: X - disabled, A - active, D - dynamic,
C - connect, S - static, r - rip, b - bgp, o - ospf, m - mme,
B - blackhole, U - unreachable, P - prohibit
# DST-ADDRESS PREF-SRC GATEWAY DISTANCE
0 ADo 0.0.0.0/0 192.168.30.1 110
1 ADo 1.2.3.1/29 192.168.30.1 110
2 DC 172.16.30.0/24 172.16.30.1 ether2-master 255
3 ADo 192.168.20.0/24 192.168.30.1 110
4 ADC 192.168.30.0/24 192.168.30.2 ether5 0